Introduction to Organic Chemistry Lecture Notes PDF

Introduction to Organic Chemistry What makes carbon so special? Why are there so many carbon-containing compounds? The answer lies in carbon’s position in the periodic table. Carbon is in the center of the second row of elements. The atoms to the left of carbon have a tendency to give up electrons...

Introduction to Organic Chemistry What makes carbon so special? Why are there so many carbon-containing compounds? The answer lies in carbon’s position in the periodic table. Carbon is in the center of the second row of elements. The atoms to the left of carbon have a tendency to give up electrons, whereas the atoms to the right have a tendency to accept electrons. Because carbon is in the middle, it neither readily gives up nor readily accepts electrons. Instead, it shares electrons. Carbon can share electrons with several different kinds of atoms, and it can also share electrons with other carbon atoms.  Schematic View of an Atom – a small dense nucleus, diameter – 10-14 - 10-15 m, which contains positively charged protons and most of the mass of the atom – an extranuclear space, diameter 10-10 m, which contains negatively charged 4 electrons  Structure of the atom ❑ The atomic number of an atom equals the number of protons in its nucleus ❑ The mass number of an atom is the sum of its protons and neutrons ❑ Isotopes have the same atomic number (i.e., the same number of protons), but different mass numbers because they have different numbers of neutrons  Electron Configuration of Atoms Electrons are confined to regions of space called principle energy levels (shells) – each shell can hold 2n2 electrons (n = 1,2,3,4......) N u mb er of Relative En ergies Electrons S hell of Electrons S hell Can Hold in Thes e Shells 4 32 h igh er 3 18 2 8 1 2 low er 6  Shells are divided into subshells called orbitals, which are designated by the letters s, p, d, f,........ – s (one per shell) – p (set of three per shell 2 and higher) – d (set of five per shell 3 and higher)..... S hell O rb itals Contain ed in Th at S hell 3 3s , 3p x , 3p y , 3p z, p lu s five 3d orbitals 2 2s , 2p x , 2p y , 2p z 1 1s 7 An orbital is a region of space where the probability of finding an electron is large 9 Degenerate orbitals are orbitals that have the same energy. The third and higher shells—in addition to their s and p orbitals—also contain five degenerate d atomic orbitals, and the fourth and higher shells also contain seven degenerate atomic orbitals. The ground-state electronic configuration of an atom describes the orbitals occupied by the atom’s electrons when they are all in the available orbitals with the lowest energy. If energy is applied to an atom in the ground state, one or more electrons can jump into a higher energy orbital. The atom then would be in an excited-state electronic configuration.  Filling the orbitals Aufbau Principle: – orbitals fill in order of increasing energy from lowest energy to highest energy 1s > 2s > 2p > 3s > 3p > 4s > 3d > 4p > 5s > 4d > 5p > 6s > 4f > 5d > 6p > 7s > 5f Pauli Exclusion Principle: – only two electrons can occupy an orbital and their spins must be paired 11  Hund’s Rule: – when orbitals of equal energy are available but there are not enough electrons to fill all of them, one electron is added to each orbital before a second electron is added to any one of them  Examples of Electron Configuration at. # 1s 2s 2px 2py 2pz 3s 3px 3py 3pz H 1 C 6 N 7 O 8 F 9 Ne 10 Cl 17 13 ❑Atomic radius The radius of an atom is the distance from the center of the nucleus to the outermost electrons 14 ❖Atomic radii vary depending on the extent of attraction between the nucleus and its electrons. The greater the attraction, the smaller the atomic radius. ❖ The most important factors that affecting the attraction force are the number of protons in the nucleus and the number of shells containing electrons. That is to say a nucleus with greater number of protons has a greater attraction for its electrons, including the outermost electrons. atomic Li Be B C N O F number 3 4 5 6 7 8 9 decreasing atomic radius H (1 shell) Li (2 shells) increasing Na (3 shells) atomic radius K (4 shells) 15 ❑Electronegativity Electronegativity is a measure of an atom’s attraction for its outer bonding electrons, especially in covalent bonded compounds. ❑Electronegativity is affected by the number of protons in the nucleus and by the number of shells containing electrons. 16  The Nature of Chemical Bonds Atoms bond together so that each atom in the bond acquires the electron configuration of the noble-gas closest it in atomic number – Octet rule: The tendency to react in ways that achieve an outer shell of eight valence electrons Types of Chemical Bonds Ionic bond Covalent bond 17  1- Ionic bond: formed between atoms widely different in EN (> 2) The bond results from one atom giving up an electron while another atom accepts the electron. The ions are held together by the electrostatic attraction of the positive and negative ions. - 18  2-Covalent Bond Formed by a sharing of two electrons by two atoms e.g H + H H H The number of shared pairs 19  Types of Covalent Bond Two types: A] non-polar covalent: ( No difference in EN) C-C , H-H, Cl-Cl B] polar covalent: is formed when the difference in the EN is < 2. + - + - C-Li, C- Cl H F C Cl 20  4- Hydrogen bonding A hydrogen bond is the electrostatic attraction between polar molecules that occurs when a hydrogen (H) atom bound to a highly electronegative atom such as nitrogen (N), oxygen (O) or fluorine (F) experiences attraction to some other nearby highly electronegative atom. The name hydrogen bond is something of a misnomer, as it is not a true bond but a particularly strong dipole-dipole attraction, and should not be confused with a covalent bond  5- Co-ordinate bond A co-ordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom. The reaction between ammonia and hydrogen chloride Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion from the hydrogen chloride to the lone pair of electrons on the ammonia molecule  Polarity of Bonds The two atoms in most cases do not share the electrons equally; the electron cloud will be denser over atom than the other. As a result of this one end of the bond is thus becomes relatively negative and the other end is relatively positive. Such a bond is said to be polar bond, or to possess polarity. The symbol  (delta) which indicate partial (+) and (-) charges is used to indicate polarity The higher difference in the electronegativity between the two atoms forming the bond, the more polar of the bond will be  Hybrid orbitals are mixed orbitals—they result from combining orbitals. The concept of combining orbitals is called orbital hybridization. If the one s and three p orbitals of the second shell are combined and then apportioned into four equal orbitals, each of the four resulting orbitals will be one part s and three parts p. This type of mixed orbital is called an (stated “s-p-three” not “s-p-cubed”) orbital. The superscript 3 means that three p orbitals were mixed with one s orbital to form the hybrid orbitals. Each orbital has 25% s character and 75% p character. 25 The superscript 3 means that three p orbitals were mixed with one s orbital to form the hybrid orbitals. Each orbital has 25% s character and 75% p character. The four orbitals are degenerate— they have the same energy. The four orbitals arrange themselves in space in a way that allows them to get as far away from each other as possible. This occurs because electrons repel each other and getting as far from each other as possible minimizes the repulsion. When four orbitals spread themselves into space as far from each other as possible, they point toward the corners of a regular tetrahedron (a pyramid with four faces, each an equilateral triangle). Each of the four bonds in methane is formed from overlap of an orbital of carbon with the s orbital of a hydrogen https://upload.wikimedia.org/wikipedia/commons/thumb/a/a5/AE4h.svg/150px-AE4h.svg.png Because the sp2 hybridized https://upload.wikimedia.org/wikipedia/commons/thumb/1/15/AE3h.svg/150px-AE3h.svg.png carbon atom is bonded to three atoms that define a plane, it is called a trigonal planar carbon.  https://upload.wikimedia.org/wikipedia/commons/thumb/4/49/AE2h.svg/150px-AE2h.svg.png To minimize electron repulsion, the two sp orbitals point in opposite directions. Consequently, the bond angles are 180°.  Acids and Bases: The Bronsted-Lowry Definition Stronger acid Stronger base than H3O+ than Cl O H O : : : : : : H O H + H O C OH H O H + :O C OH : carbonic acid stronger acid bicarbonate ion 31 than H2CO3 stronger base than H2O  Lewis theory of acids and bases: Lewis offered new definitions for the terms “acid” and “base.” He defined an acid as a species that accepts a share in an electron pair and a base as a species that donates a share in an electron pair. All proton-donating acids fit the Lewis definition because all proton-donating acids lose a proton and the proton accepts a share in an electron pair  The Nature of Chemical Reactions The chemical reaction between two substances involves the breaking of already existing bond and formation of a new one. Energy in chemical reaction as energy is evolved when a bond is formed but when it breaks it needs the same amount of energy 37 Types of chemical reactions 1- Substitution reaction Is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved  2- Addition Reactions Is a reaction in which atoms or groups are attached to the molecule without displacement of any other atoms or groups. Only unsaturated compounds give addition reaction 3- Elimination Reactions Is a reaction in which atoms or groups are detached from the molecule without displacement of any other atoms or groups. This reaction is reverse of addition reaction Chemical formula in organic chemistry Lewis formulas are useful for keeping tack of bonding electrons, but organic chemists rarely use true Lewis formulas. Let us consider the types of chemical formulas that are more frequently encountered. 1- An empirical formula tells us the types of atoms and their numerical ratio in a molecule. For example, a molecule of ethane contains carbon and hydrogen atoms in a ratio of 1 : 3; the empirical formula is CH3. 2- A molecular formula tells us the actual number of each type of atom in a molecule, not the ratio. The molecular formula for ethane is C2H6. 3- A structural formula shows the structure of a molecule, that is the order of attachment of the atoms. In order to explain or predict the chemical reactivity, we need to know the structure of a molecule. Therefore, structural formulas are the most useful of the different types of formulas. H H H C C H CH3 C2H6 H H empirical formula molecular formula structural formula (for ethane) Structural Formulas REPRESENTATION OF STRUCTURAL FORMULAS  Types of bond fission (dissociation) The breaking of a simple covalent bond can take place in two different ways 1- Heterolysis (Heterolytic Fission): In this type of fission the bond is broken unsymmertically, i.e. the two electrons are retained by one atom. Such fissure results in the formation of ions. A: +B A : B A + :B 2- Homolysis (Homolytic Fission): This type involves the symmetrical breaking of the shared electron pair, one electron being retained by each atom. hemolytic or free radical reactions A:B A. + B. Depending on the electronegativity of the atom attached to carbon in comparing with carbon  Types of Reagents 1) Nucleophiles or nucleophilic reagents (electron donors): are nucleus loving reagents which in a reaction donate electrons to share by its with the center attached N: + R :X R: N + :X A- Negatively charged anions as OH-, CN-, RO- OH + R X R OH + X B- Neutral molecules having lone pair of electrons R3N: H3N: H2O: R2O: R2S : 2) Electrophiles or Electrophilic reagents (electron acceptors): electron loving reagents Positively charged cations Such as, H+, NO2+, X+, CH3+ Neutral molecules (electron deficient) Such as FeCl3, AlCl3, C=O, HYDROCARBONS They are compounds whose molecules contain only carbon and hydrogen atoms. There are two main classes: aliphatic and aromatic compounds. Within the aliphatic class there are both saturated (alkanes) and unsaturated (alkenes and alkynes) hydrocarbons. Methane (CH4) and ethane (C2H6) are hydrocarbons and they also belong to a subgroup of hydrocarbons known as alkanes (paraffins) whose members do not have multiple bonds between carbon atoms, they have the general formula CnH2n+2. If the carbon atoms are joined in ring, they are called cycloalkanes and have the general formula CnH2n. Both alkanes and cycloalkanes are called saturated hydrocarbons, meaning saturated with hydrogen.  Some compounds take the name from the chemists who first discovered them. These names called trivial names or common names. After that with the specter of unlimited number of organic compounds, organic chemists are systematizing organic nomenclature to correlate the names of compounds with their structures. The system of nomenclature that has been developed is called the IUPAC system, for the international Union of Pure and Applied Chemistry No. of Structure Name carbons 1 CH4 Methane 2 CH3CH3 Ethane 3 CH3CH2CH3 Propane 4 CH3CH2CH2CH3 Butane 5 CH3CH2CH2CH2CH3 Pentane 6 CH3CH2CH2CH2CH2CH3 Hexane 7 CH3CH2CH2CH2CH2CH2CH3 Heptane 8 CH3CH2CH2CH2CH2CH2CH2CH3 Octane 9 CH3CH2CH2CH2CH2CH2CH2CH2CH3 Nonane 10 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 Decane Branched-chain alkanes If we remove one hydrogen atom from an alkane, we obtain what is called an alkyl group. These alkyl groups have names that end in –yl. For alkanes with more than two carbon atoms, more than one derived group is possible, for example: Alkane Alkyl group CH3-H Methane CH3- Methyl CH3CH2-H Ethane CH3CH2- Ethyl CH3CH2CH2-H Propane CH3CH2CH2- Propyl CH3CH2CH2CH2-H Butane CH3CH2CH2CH2- Butyl Butane CH3CH2CH3 Propane CH3 CH3 CH3 CH3 CH3CH2CH2 H3C CH CH3CH2CH2CH2 H3C C CH2 CH3CH2C H3C C Propyl H H CH3 Isopropyl n-Butyl Isobutyl sec-Butyl tert- butyl Rules of nomenclature of alkanes 1. Determine the number of carbons in the longest continuous carbon chain. This chain is called the parent hydrocarbon. The longest continuous chain is not always a straight chain; sometimes you have to “turn a corner” to obtain the longest continuous chain. 2- Number the longest chain (parent), beginning with the end of the chain nearer the substituent Use the numbers obtained by application of rule 2 to designate the location of the substituent group 4- When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain (parent). The substituent groups should be listed alphabetically (ethyl before methyl), disregard multiplying prefixes such as “di” and”tri” 5- When two substituents are present on the same carbon atom, use that number twice. CH3 CH3CH2CHCH2CHCH3 CH3CH2 C CH2CH2CH3 CH3 CH3 CH2 2,4-dimethylhexane CH3 3-Ethyl-3-methylhexane 6- If more than one substituent is attached to the parent hydrocarbon, the chain is numbered in the direction that will result in the lowest possible number in the name of the compound. The substituents are listed in alphabetical.If two or more substituents are the same, the prefixes “di,” “tri,” and “tetra”are used to indicate how many identical substituents the compound has CH3 CH3 CH3 CH3CH CHCH3 CH3CHCHCHCH3 CH3CCH2CCH3 CH3 CH3 CH3 CH3 CH3 CH3 2,3-Dimethylbutane 2,3,4-Trimethylpentane 2,2,4,4-Tetramethylpentane - When branching first occurs at an equal distance from either end of the longest chain, choose the name that gives the lower number at the first point of difference. 6 5 4 3 2 1 CH3 CH CH2 CH CH CH3 CH3 CH3 CH3 2,3,5-Trimethylhexane (not 2,4,5-trimethylhexane) 7- If a compound has two or more chains of the same length; the parent hydrocarbon is the chain with the greatest number of substituents CH3CH2CHCH2CH2CH3 H3CHC CH3 3-ethyl-2-methylhexane (two substituents) 7 6 5 4 3 2 1 CH3CH2 CH CH CH CH CH3 CH3 CH2 CH3 CH3 CH2 CH3 2,3,5-Trimethyl-4-propylheptane (four substituents)  Rules for naming cycloalkanes: 1) Monocyclic compounds: Cycloalkanes with only one ring are named by attaching the prefix cyclo- to the names of the alkanes possessing the same number of carbon atoms. For example Cyclopropane cyclopentane cyclohexane 2- In the case of a cycloalkane with an attached alkyl substituent, the ring is the parent hydrocarbon unless the substituent has more carbon atoms than the ring. In that case, the substituent is the parent hydrocarbon and the ring is named as a substituent. There is no need to number the position of a single substituent on a ring. CH3 CH3 CH2CH3 CH2CH3 3 1 2 2 1 CH3 4 3 CH CH3 Br Ethylcyclohehane 1-Isopropyl-3-methylcyclohexane 4-Bromo-2-ethyl-1-methylcyclohexane (not 1-ethyl-5-methylcyclohexane) (not 1-chloro-3-ethyl-4-methylcyclohexane) 3- If the ring has two different substituents, they are cited in alphabetical order and the number 1 position is given to the substituent cited first. CH3 H3C CH2CH3 1-ethyl-2-methylcyclopentane CH3 1,3-dimethylcyclohexane Types of Carbons: Carbons are classified according to the number of other carbons attached to the carbon in question. Primary (1°) carbons have one other carbon attached. Secondary (2°) carbons have two other carbons attached. Tertiary (3°) carbons have three other carbons attached. Quaternary carbons have four other carbons attached. Tertiary carbon Secondary carbon CH3 Primary carbon H3C C CH2 CH CH2 CH3 Quaternary carbon CH3 CH3 Classification of Hydrogen atoms: The hydrogen atoms of an alkane are classified on the basis of the carbon atom to which they are attached. A hydrogen atom attached to a primary carbon is primary (1o) hydrogen, and so on. 2o hydrogen atom 1o hydrogen atom CH3 H3C H2C CH CH3 3o hydrogen atom 1o hydrogen atom 2-Methylbutane Physical Properties of alkanes:- Methane, ethane, propane, and butane are gases; pentane to heptadecane are liquids; the homologues larger than heptadecane are solids. The boiling points and melting points of alkanes increase with molecular weight. Branching reduces the boiling point, the more branching the lower the boiling point Alkanes are nonpolar so are immiscible with water, they are soluble in most organic solvents. Synthesis of alkanes and cycloalkanes: I- Hydrogenation of alkenes: Ni / ethanol propene 25o / 50 atm propane Ni / ethanol 25o / 50 atm 2-methylpropene 2-methylpropane Pt / ethanol cyclohexene 25o / 50 atm cyclohexane II- Reduction of alkyl halide: Alkyl halide reduction via zinc / acid Most alkyl halides react with zinc and aqueous acid to produce an alkane. In these reactions zinc atoms transfer electrons to the carbon atom of the alkyl halide. Therefore, the reaction is a reduction of the alkyl halide. H Zn Br sec-butyl bromide butane (2-bromobutane) Br H Zn isopentane isopentyl bromide (2-methylbutane) (1-bromo-3-methylbutane) III. Hydrolysis of a Grignard Reagent Alkyl or aryl magnesium halide (R-MgX) are called Grignard reagents or organometallic compounds. Grignard reagent on double decomposition with water gives alkane i) R—X + Mg → RMgX (Grignard reagent) ii) RMgX + H2O → RH + Mg(OH)X  Reactions of alkanes: Alkanes and cycloalkanes are relatively unreactive compared with organic compounds containing functional groups. The main reasons for unreactivity of alkanes are: 1- The C-C and the C-H bonds are strong bonds. 2- Neither H nor C atoms in the paraffins have any unshared electrons. or any orbitals in their valency shells not completely filled. 3- No dipole moment between any C-C or C-H bonds.  Reactions of Alkanes 1. Halogenation Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, shown below, provides a simple example of this reaction. CH4 + Cl2 + energy ——> CH3Cl + HCl The following facts must be accommodated by any reasonable mechanism for the halogenation reaction 1. The reactivity of the halogens decreases in the following order: F2 > Cl2 > Br2 >I2. 2. We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive. 3. Chlorinations and brominations are normally exothermic. 4. Energy input in the form of heat or light is necessary to initiate these halogenations. 5. Halogenation reactions may be conducted in either the gaseous or liquid phase The mechanism is illustrated in the following chart including 3 steps: 1-Initiation step 2-Propagation step 3-Termination step Alkenes Nomenclature Many older names for alkenes are still in use as ethylene, propylene and isobutylene. CH3 H2C CH2 H2C CH CH3 H2C C CH3 common: ethylene propylene isobutylene IUPAC: ethene propene 2-methylpropene  Determine the parent name by selecting the longest chain that contains the double bond and replace the end of the name of the parent alkane of identical length from –ane to –ene. Number the chain so as to include both carbon atoms of the double bond, and begin numbering at the end of the chain nearer the double bond. Designate the location of the double bond by using the number of the first atom of the double bond as a prefix:. ‫ م‬09:37 28/07/2024 4  Indicate the location of the substituent groups by the numbers of the carbon atoms to which they are attached. 2 3 4 2 3 4 5 6 1 1 2-methyl-2-butene 2,5-dimethyl-2-hexene (not 3-methyl-2-butene) (not 2,5-dimethyl-4-hexene) 2 4 6 3 1 5 2 1 4 Cl 3 5,5-dimethyl-2-hexene 1-chloro-2-butene ‫ م‬09:37 28/07/2024 5  Number substituted cycloalkenes in the way that gives the carbon atoms of the double bond the 1 and 2 positions and that also gives the substituent groups the lower numbers at the first point of difference. 1 CH3 6 2 1 5 2 H3C 5 4 3 CH3 4 3 1-methylcyclopentene 3,5-dimethylcyclohexene (not 2-methylcyclopentene) (not 4,6-dimethylcyclohexene) Priority of functional groups in iupac nomenclature.  Name compounds containing a double bond an alcohol group as alkenols (or cycloalkenol) and give the alcohol carbon the lower number. Synthesis of Alkenes 1- Debromination of vic-dibromides: Vicinal (or vic) dihalides are dihalo compounds in which the halogens are situated on adjacent carbon atoms. The name geminal (or gem) dihalides is used for those dihalides where both halogen atoms are attached to the same carbon. X C C C C X X X vic-dihalide gem-dihalide  Br NaI acetone Br 2-pentene 2,3-dibromopentane Br Zn ethanol Br 3,4-dibromohexene 3-hexene 2- Dehydration of alcohols: Heating of most alcohols with a strong acid causes them to dehydrate (to lose a molecule of water) and form an alkene. The reaction is an elimination reaction and is favored at higher temperatures. H3C CH3 20% H2SO4 H3C C OH H3C C + H2O o 85 C CH3 CH2 tert-butyl alcohol 2-methylpropene (84%) Step 1: In this step: a proton is rapidly transferred from the acid to one of the unshared electron pairs of the oxygen atom of the alcohol. In dilute sulfuric acid, the acid is a hydronium ion but in concentrated sulfuric acid, the proton donor is sulfuric acid itself. This step is characteristic of all reactions of an alcohol with a strong acid. CH3 H CH3 H : : : : H3C C O H + H O: H3C C O H + H O H : CH3 H CH3 Protonated alcohol or alkyloxonium ion Step 2: The carbon-oxygen bond breaks heterolytically. The bonding electrons depart with the water molecule and leave behind a carbocation. The carbocation is, of course highly reactive because the central carbon atom has only six electrons in its valence level not eight. CH3 H CH3 : : H3C C O H H3C C + H O H : CH3 CH3 A carbocation Finally in step 3, the carbocation transfers a proton to a molecule of water. The result is the formation of a hydronium ion and an alkene. Step 3: In this step, also it is an acid-base reaction, any one of the nine protons available at the three methyl groups can be transferred to a molecule of water. The electron pair that bonded the hydrogen atom to the carbon atom in the carbocation becomes the second bond of the alkene. Notice that this step restores an octet of electrons to the central carbon atom. H H C H CH2 H H H3C C + :O H H3C C + H O H : : CH3 CH3 2-Methylpropene 3- Dehydrohalogenation of alkyl halides (haloalkanes): C2H5O-Na+ H3C CH CH3 H2C CH CH3 C2H5OH (79%) Br 55oC CH3 CH3 - + C2H5O Na H3C C CH3 H2C C CH3 C2H5OH (100%) Br 55oC (CH3)3CO-K+ CH3(CH2)15CH2CH2Br CH3(CH2)15CH CH2 (CH3)3COH (85%) 40oC Reactions of alkenes (olefins) The characteristic linkage of olefins is covalent and consists of (a)-a strong -bond (2 - electrons), (b)-a weaker -bond (2 -electrons). We have seen that a bond is weaker than a bond. The bond, therefore, is the bond that is most easily broken when an alkene undergoes a reaction. an alkene is an electron-rich molecule, it acts as a nucleophile. We can, therefore, predict that an alkene will react with an electrophile, and, in the process, the bond will break (Addition reaction). So, if a reagent such as hydrogen bromide is added to an alkene, the alkene will react with the partially positively charged hydrogen of hydrogen bromide and a carbocation will be formed. In the second step of the reaction, the positively charged carbocation (an electrophile) will react with the negatively charged bromide ion (a nucleophile) to form an alkyl halide  NATURE OF ADDITION REACTIONS TO ALKENES: All addition reactions to olefins are electrophilic processes i.e. the reaction is initiated by an electrophilic (electron seeking) reagent. In an addition reaction of an alkene, the  bond is broken, and its pair of electrons is used in the formation of two new σ bonds. In each case, the sp2 hybridized carbon atoms are rehybridized to sp3. Compounds containing  bonds are usually of higher energy than comparable compounds containing only σ bonds. Consequently, an addition reaction is usually exothermic.  nucleophile electrophile H+ Nu : H two electrons C C C C in the  bond a carbocation Nu H H C C or C C Nu Addition Reactions 1) Hydrohalogenation (addition of hydrogen halides). Hydrogen halides (HI, HBr, HCl and HF) add to the double bond of alkenes; General reaction C C + HX C C H X Specific example H3CHC CHCH3 + HBr CH3CH2CHCH3 Br But-2-ene 2-Bromobutane The order of reactivity of the hydrogen halides is; HI > HBr > HCl> HF. Step 1 H.. Slow.. + + X.... H X C C.. C C.... the  electrons of the alkene form a bond with a proton from HX to form a carbocation and a halide ion. Step 2 H H.. Fast X +...... C C C C X...... The halide ion reacts with the carbocation by donating an electron pair, the result is an alkyl halide. The addition of hydrogen halide to an unsymmetrical alkene, such as propene, may occur in two ways. CH2 CHCH3 + HBr CH3 CHCH3 Br propene 2-bromopropane BrCH2CH2CH3 1-bromopropane (little) Markovnikov’s rule In the addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms. The addition of HBr to propene is an illustration. CH2 CHCH3 + HBr CH3 CHCH3 Br propene 2-bromopropane BrCH2CH2CH3 1-bromopropane (little) Anti-Markovnikov’s Addition: Markovnikov’s rule is often not obeyed upon addition of HBr to an alkene in the presence of small amount of a peroxide. This may be explained by that peroxides react with hydrogen bromide (HBr) to give water and bromine atoms (free radicals). Bromine atoms (free radicals) then add to the terminal carbon to give the more stable a carbon skeleton of free radical character shown below. This carbon free radical reacts with HBr to regenerate free radical bromine and so the reaction is continued.. H O 2 2+H B r H O 2+B r 2) Addition of bromine and chlorine (halogenation): Alkenes react rapidly with chlorine and bromine to form vicinal dihalides. Examples - 9 oC CH3CH CHCH3 + Cl2 CH3CH CHCH3 Cl Cl (100%)  Step 1.. C C C C + Br........ Br  + Br........  − Br Bromonium ion bromide ion...... A bromine molecule becomes polarized as it approaches the alkene. The polarized bromine molecule transfers a positive bromine atom (with six electrons in its valence shell) to the alkene resulting in the formation of a bromonium ion. Step 2.. Br...... C C + Br C C...... Br.... Br...... Bromonium ion bromide ion vic- Dibromide A bromide ion attacks at the back side of one carbon (or the other) of the bromonium ion in an SN2 reaction, causing the ring to open and resulting in the formation of a vic-dibromide. 3) Addition of water (hydration): The acids most commonly used to catalyze the hydration of alkenes are dilute H2SO4 and H3PO4. The addition of water to the double bond follows Markovnikov’s rule. General reaction H3O+ C C + HOH C C H OH 4) Addition of Hydrogen (Hydrogenation or Reduction): It is carried out by molecular hydrogen (H2) in the presence of catalysts such as nickel (Ni), platinum (Pt) or palladium (Pd) and known as catalytic hydrogenation. The product that results from the addition of hydrogen to an alkene is an alkane. Ni, Pd, or H2C CH2 + H2 H3C CH3 o Pt at 25 C 5) Oxidation of alkenes: Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized. Several oxidizing agents, e.g. KMnO4, K2Cr2O7, O3, and peroxy acids may be used. (a) Oxidation by potassium permanganate Potassium permanganate can be used to oxidize alkenes to 1,2-diols called glycols. OH, H2O CH2 CH2 + KMnO4 CH2 CH2 cold OH OH Ethene 1,2-ethanediol (ethylene glycol) CH2 OH, H2O CH2 CH2 CH2 + KMnO4 1,2-ethanediol (ethylene glycol) Ethene cold OH OH (1) OsO4, pyridine CH3CH CH2 CH3CH CH2 (2) Na2SO3/H2O or NaHSO3/ H2O OH OH Propene 1,2-propanediol (propylene glycol) OH C C C C C C + MnO2 H2O + O O several steps OH OH O O Mn Mn O O O O Or NaHSO3 C C C C C C + Os Pyridine H2O + O O OH OH O O Os Os O O O O an osmate ester  Oxidative cleavage of alkenes Alkenes with monosubstituted carbon atoms are oxidatively cleaved to salts of carboxylic acids, by hot basic potassium permanganate solutions. O O KMnO4, OH , H2O 2 H+ 2 H3C C H3C HC CH CH3 H3C C heat O OH 2-butene (cis or trans) acetate ion acetic acid (B) Oxidations by using peroxyacids (Epoxidations): Epoxides are cyclic ethers, with three-membered rings. In IUPAC nomenclature epoxides are called oxiranes. The simplest epoxide has the common name ethylene oxide. H2C CH2 C C IUPAC name : Oxirane An epoxide O.... O common name: ethylene oxide.... The most widely used method for preparing epoxides is the reaction of alkene with an organic peroxy acid (called simply peracid) a process that is called epoxidation. CH3 CH3 O O H3C CH CH CH3 epoxidation HC CH + R' C O OH + R' C OH 2-Propene O Peroxy acid An epoxide (2,3-Dimethyloxirane) (C) Oxidation by ozone (ozonolysis of alkenes). A more widely used method for locating the double bond of an alkene involves the use of ozone (O3). Ozone reacts vigorously with alkenes to form unstable compounds called initial ozonides, which rearrange spontaneously to form compounds known as ozonides. H3C CH3 H3C CH3 (1) O3, CH2Cl2, - 78oC C C C O + O C H3C H (2) Zn/ HOAc H3C H 6) Oxymercuration-demercuration of alkenes: It is a useful method for synthesizing alcohols from alkenes. Alkenes react with mercuric acetate in the presence of water to produce (hydroxyalkyl)mercury compounds, which upon reduction with sodium borohydride yield alcohols. This method of hydration is fast and convenient, takes place under mild conditions and gives high yields of alcohol. ❑ This method also is highly regioselective. The net orientation of the addition of the elements of water, H- and -OH, is in accordance with Markovnikov’s rule. 1) Hg(OAc)2 CH3(CH2)2CH CH2 NaHBH4 CH3(CH2)2CHCH2 THF-H2O OH OH 1-Pentene HgOAc CH3(CH2)2CH CH3 + Hg OH 2-Pentanol (93%) 2) CH3 H3C OH H3C OH Hg(OAc)2 HgOAc NaHBH4 + Hg THF-H2O H OH 1-Methylcyclopentene 1-methylcyclopentanol + CH3COOH 7) Hydroboration oxidation: The addition of a compound containing a hydrogen-boron bond, H-B (called a boron hydride), to an alkene consider a starting point for the preparation of number of organic compounds, this addition called hydroboration. Hydroboration can be carried out using the boron hydride (B2H6) called diborane. When diborane dissolves in Tetrahydrofuran (THF), each B2H6 dissociates to produce two molecules of a complex between BH3 (borane) and THF. BH3 is a lewis acid, it + O BH3 accepts an electron pair.. B2H6 2 O...... 2 from oxygen atom of THF Diborane THF THF : BH3 When a 1-alkene such as propene is treated with a solution containing THF:BH3 complex, the boron hydride adds successively to the double bonds of three molecules of the alkene to form a trialkylborane. In each addition step the boron atom becomes attached to the less substituted carbon atom of the double bond, and a hydrogen atom is transferred from the boron atom to the other carbon atom of the double bond. Thus, hydroboration is regioselective and it is anti- Markovnikov (the hydrogen atom becomes attached to the carbon atom with fewer hydrogen atoms). More substituted Less substituted CH3CH CH2 CH3CH CH2 CH3CH CH2 (CH CH CH ) BH BH2 3 2 2 2 + H BH2 H CH3CH CH2 (CH3CH2 CH2) 3 B Tripropylborane Hydroboration-oxidation, by contrast, yields 1-hexanol; (1) THF: BH3 CH3CH2CH2CH2CH CH2 CH3CH2CH2CH2CH2CH2OH (2) H2O2, OH 1- Hexene 1- Hexanol (90%)  Examples of hydroboration-oxidation : CH3 CH3 (1) THF: BH3 CH3 C CHCH3 CH3 C CHCH3 (2) H2O2, OH H OH 2-Methyl-2-butene 3-methyl-2-butanol (59%) (1) THF: BH3 CH3 CH3 (2) H2O2, OH H H 1-Methylcyclopentene OH trans-2-methylcyclopentanol (86%) Alkynes  Examples: H C C H C3HH2CC CCH3 HC CCH2CH2OH Acetylene 2-Pentyne 3-Butyn-1-ol Ethyne H2CC CCH2 H3CC CCH3 CH3 CH3 2-Butyne 3-Hexyne Preparation of alkynes: Dehydrohalogenation of dihalides: Dehydrohalogenation of the vic-dibromide was carried out using a strong base (sodium amide NaNH2), the dehydrohalogenation occurs in two steps. The first step yields a bromoalkene. The vic-dibromide first can be prepared from alkenes upon treatment with bromine. H H H NaNH2 NaNH2 RCH CHR + Br2 RC CR RC CR RC CR Br Br Br Alkene vic-Dibromide bromoalkene Alkyne  Br2 NaNH2 CH3CH2CH CH2 CH3CH2CH CH2Br CH3CH2CH CHBr CCl4 mineral oil Br 110-160oC + CH3CH2C CH2 Br NaNH2 mineral oil 110-160oC CH3CH2C CH Reactions of alkynes: The alkynes show the same chemical behaviour as the alkenes. Alkynes add the same electrophilic reagents as in the case of alkene and its homologues. In general, the triple bond is more reactive (i.e. more strongly nucleophilic) and in consequence will add some weakly electrophilic reagents that do not add readily, to the double bond of alkene. The additions occur in two stages and since the first stage takes place more readily than the second, it is often possible, by the use of one molecular ratio of the reagent, to control the reaction so as to give the first stage product. The mechanism of the additions is presumably the same as with the alkenes, the initial step involving reaction between the nucleophilic unsaturated carbon atom and the electrophilic reagent atom. 1) Addition of Br2 and Cl2 to alkynes: Alkynes show the same kind of reactions toward Cl2 and Br2 that alkenes do. They react by addition. However, with alkynes, the addition may occur once or twice, depending on the number of molar equivalents of halogen (one or two equivalents) we employ. Br Br Br Br2 Br2 C C C C C C CCl4 CCl4 Br Br Br Alkyne Dibromoalkene Tetrabromoalkane Cl Cl Cl Cl2 Cl2 C C C C C C CCl4 CCl4 Cl Cl Cl Alkyne Dichloroalkene Tetrachloroalkane 2) Hydrohalogenation reactions: Alkynes react with hydrogen chloride, and hydrogen bromide to form haloalkenes or geminal dihalides, depending on, how many molar equivalents (one or two) of the hydrogen halide are used. Both additions are regioselective and follow Markovnikov’s rule: X H X HX HX C C C C C C H H X Alkyne Haloalkene gem-Dihaloalkane Specific example Br HBr C4H9 C CH2 HBr C4H9 C CH C4H9 C CH3 Br Br 1-Hexyne 2-Bromo-1-hexene 2,2-Dibromohexane  3) Hydration reactions: Alkynes add water readily when the reaction is catalyzed by strong acids and mercuric (Hg2+) ions. The vinylic alcohol that is initially produced is usually unstable, and it rearrange to a ketone. OH H O H2O C C C C C C H2SO4, H H HgSO4 Alkyne A vinylic alcohol (unstable) Ketone These rearrangements are known as keto-enol tautomerization because the vinylic alcohols are often called enols and the product is ketones. 4) Hydrogenation reactions (Reduction) Depending on the conditions and the catalyst employed,one or two molar equivalents of hydrogen (H2) will add to a carbon-carbon triple bond as follows: In the presence of catalysts such as nickel (Ni), platinum (Pt) or palladium (Pd) and known as catalytic hydrogenation. The product that results from the addition of hydrogen to an alkyne is an alkane. H2, Pt H2, Pt H3C C C CH3 CH3CH CHCH3 CH3CH2 CH2CH3 Hydrogenation can be stopped at the intermediate alkene stage by the use of modified catalyst such as Lindlar’s Catalyst (a palladium on calcium carbonate combination). As a result, this method affords a stereoselective synthesis of cis alkene from alkyne. Reduction by sodamide NaNH2 gives trans alkene H3CH2C CH2CH3  H2, PdCaCO3 (lindlar s catalyst) C C H3CH2C C C CH2CH3 quinoline, lead acetate (syn addition) H H  5) Oxidative cleavage of alkynes: Treating alkynes with ozone or with basic KMnO4, leads to cleavage at the carbon-carbon triple bond. The products are carboxylic acids. (1) O3 R C C R' RCOOH + R'COOH (2) Zn/HOAc (1) KMnO4, OH Or R C C R' RCOOH + R'COOH (2) H+