Chapter 6 Hydrocarbons (Alkanes, Alkenes & Alkynes) PDF

FBC0025/ FCC0025- Chemistry II CHAPTER 6 Hydrocarbons (Alkanes, Alkenes & Alkynes) Prepared by: NDMR, NIK Slide Content 6.1: Alkanes 6.2: Alkenes 6.3: Alkynes 6.4: Oxidat...

FBC0025/ FCC0025- Chemistry II CHAPTER 6 Hydrocarbons (Alkanes, Alkenes & Alkynes) Prepared by: NDMR, NIK Slide Content 6.1: Alkanes 6.2: Alkenes 6.3: Alkynes 6.4: Oxidation and Reduction of Hydrocarbons SCOPE Part 1: Alkanes Introduction Nomenclature Physical Properties of Alkanes Reaction of alkanes Oxidation of Alkanes Combustion of Alkanes  Learning Outcomes (Part 6.1) At the end of this lecture, students should be able to: define and classify hydrocarbons into: (i) aliphatic and aromatic (ii) saturated and unsaturated draw structure and name alkane compounds according to IUPAC rules for: (i) straight chain and branched alkanes (parent chain ≤ C10) (ii) cyclic alkanes (C3 – C6) (iii) alkyl groups explain the following physical properties: (i) boiling points of alkanes based on molecular weight (ii) boiling points of isomeric alkanes (iii) boiling points of alkanes and cycloalkanes (iv) solubility in water and organic solvents write equations for the combustion of alkanes in excess oxygen and limited oxygen specify the types of reactions for alkanes  Recall Chapter 5 6.1.1 Introduction to alkanes Alkanes are aliphatic hydrocarbons having C—C and C—H  bonds. They can be categorized as acyclic or cyclic. Acyclic alkane Cyclic Acyclic alkanes contain carbon atoms arranged in a chain Cycloalkanes contain carbons joined in one or more (linear and branched) rings Saturated hydrocarbon with general formula of CnH2n+2. They Hydrocarbon with one degree of unsaturation with are also called saturated hydrocarbons because they general formula of CnH2n. (Fewer H atoms than an have the maximum number of hydrogen atoms per carbon. acyclic alkane with the same number of carbons.) Example Example  6.1.1 Introduction to alkanes Acyclic alkanes Linear alkanes, homologous All C atoms in an alkane are surrounded by four series of alkanes groups and all bond angles are 109.50. The 3-D representations and ball-and-stick models for these alkanes indicate the tetrahedral geometry around each C atom.  6.1.1 Introduction to alkanes Cycloalkanes Cycloalkanes contain carbon atoms arranged in a ring. Simple cycloalkanes are named by adding the prefix cyclo- to the name of the acyclic alkane having the same number of carbons.  6.1.2 Nomenclature According to IUPAC system, the name of every organic molecule has 3 parts: 1. The parent name indicates the number of carbons in the longest continuous chain. 2. The suffix indicates what functional group is present. 3. The prefix tells us the identity, location, and number of substituents attached to the carbon chain.  6.1.2 Nomenclature of alkanes Prefix + Parent name + Suffix 1. Alkanes do not have functional groups. Their suffix is -ane. 2. The longest chain is the parent chain Naming of acyclic alkane and cyclic alkane has the same rules, but cyclic alkane has the word ‘cyclo’ added before the parent name. 3. Any substituent on the parent chain should be added in the prefix.  6.1.2 Nomenclature of alkanes The prefix Carbon substituents bonded to a long carbon chain are called alkyl groups. An alkyl group is formed by removing one H atom from an alkane. To name an alkyl group, change the –ane ending of the parent alkane to –yl. Example: methane (CH4) becomes methyl (CH3-). Ethane (CH3CH3) becomes ethyl (CH3CH2-). Naming three- or four-carbon alkyl groups is more complicated because the parent hydrocarbons have more than one type of hydrogen atom. For example, propane has both 10 and 20 H atoms, and removal of each of these H atoms forms a different alkyl group with a different name, propyl or isopropyl.  6.1.2Nomenclature 6.1.2 Nomenclatureof alkanes Steps for naming ACYCLIC alkane- Step 1 1. Find the parent carbon chain and add the suffix. a) Rule: parent chain is the longest chain. it does not matter if the chain is straight or it bends.  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 1 b) if there are two chains of equal length, pick the chain with more substituents. In the following example, two different chains in the same alkane have seven C atoms. We circle the longest continuous chain as shown in the diagram on the left, since this results in the greater number of substituents.  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 2 2. Number the atoms in the carbon chain. a) Rule: the first substituent has the lowest number.  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 2 b) If the first substituent is the same distance from both ends, number the chain to give the second substituent the lower number.  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 2 c) When numbering a carbon chain results in the same numbers from either end of the chain, assign the lower number alphabetically to the first substituent.  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 3 3. Number and name the substituent in the prefix Rules a) Name the substituents as alkyl groups.Every carbon belongs to either the longest chain or a substituent, not both. Each substituent needs its own number b) If two or more identical substituents are bonded to the longest chain, use prefixes to indicate how many: di- for two groups, tri- for three groups, tetra- for four groups, and so forth. c) Alphabetize the names of substituents. For example, comparing methyl and ethyl, ethyl will be named first in the prefix since it starts with alphabet ‘e’. d) Indicate the position of substituent by the carbon it is attached to. Numbers are separated by comma and numbers and letters are separated by hyphens. In alphabetizing, the prefixes - di, tri, tetra, sec- and tert- are ignored - iso, neo and cyclo are included  6.1.2 Nomenclature of alkanes Steps for naming ACYCLIC alkane- Step 4 4. Combine substituent names and numbers + parent and suffix. Precede the name of the parent by the names of the substituents. Alphabetize the names of the substituents, ignoring all prefixes except iso, as in isopropyl and isobutyl. Precede the name of each substituent by the number that indicates its location. Separate numbers by commas and separate numbers from letters by hyphens. The name of an alkane is a single word, with no spaces after hyphens and commas.  6.1.2 Nomenclature of alkanes Summary Nomenclature of branched chain alkanes Rule 1 Find the longest continuous chain of C atoms (determine the parent name for alkanes) Rule 2 If there are two chain of equal length as the parent chain, choose the parent with the greater number of substituent Rule 3 Number the longest continuous chain beginning with the end of the chain nearer the substituent Rule 4 If the branching occurs at equal distance from either end of the longest chain, the C atoms are numbered from the end of the chain which gives the substituent the lowest possible number Rule 5 Identify the name of the substituent - Other atom/group attached to the longest chain are called the substituent group  6.1.2 Nomenclature of alkanes Summary IUPAC nomenclature for alkanes Rule 1 The position and the name of the substituent must be written in front of the parent chain Rule 2 Specify the locations of the substituent using the numbers assigned to the C atom of parent chain Rule 3 Use: hypens (-) to separate number and word Example: 2-methylhexane Use: commas (,) to separate number and number Example: 2,2-dimethyloctane Rule 4 If two substituents are present, give each substituent a number corresponding to its location on the longest chain (parent chain) (i) The substituent should be listed alphabetically (ii) In alphabetizing, the prefixes - di, tri, tetra, sec- and tert- are ignored - iso, neo and cyclo are included Rule 5 If two or more identical substituents are present, use prefixes number of identical substituent prefix 2 di 3 tri 4 tetra  6.1.2 Nomenclature of alkanes Exercises 1. Name the following compounds according to the IUPAC nomenclature a) c) Ans: 1. a) 2,4,4-trimethylhexane b) 4-isopropyl-3-methyloctane c) 3,4-dimethyl-5-propyloctane d) 4-ethyl-3-isopropyl-2,2- dimethylhexane 2. b) d) a) b) 2. Write the structural formula for the following hydrocarbon a) 3-methylpentane c) 3-ethyl-2,2,4-trimethylhexane c) d) b) 1,2-dichloro-3-methylbutane d) 4-ethyl-2,2-dimethylhexane  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane Cycloalkanes are named by using similar rules, but the prefix cyclo- immediately precedes the name of the parent. 1. Find the parent cycloalkane.  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane- step 1 1. Find the parent cycloalkane.  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane- step 2 2. Name and number the substituents. Rule: a) No number is needed to indicate the location of a single substituent. b) For rings with more than one substituent, begin numbering at one substituent and proceed around the ring to give the second substituent the lowest number.  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane- step 2 & 3 c) With two different substituents, number the ring to assign the lower number to the substituents alphabetically. 3. Combine all the name (prefix+ parents + suffix)  6.1.2 Nomenclature of alkanes Steps for naming CYCLIC alkane- EXAMPLE  6.1.2 Nomenclature of alkanes Steps for naming alkane- special case Note the special case of an alkane composed of both a ring and a long chain. If the number of carbons in the ring is greater than or equal to the number of carbons in the longest chain, the compound is named as a cycloalkane. Naming compounds containing both a ring and a long chain of carbon atoms  6.1.2 Nomenclature of alkanes Summary IUPAC nomenclature of cycloalkanes Step 1 Name the parent of cycloalkanes Step 2 Number the cyclic compounds Rule 1 If only one substituent is present, it is not necessary to designate its position. Rule 2 If two substituents are present, number carbon in the ring beginning with the substituent according to the alphabetical order followed by giving a possible lowest number to the next substituent. Rule 3 When three or more substituents are present, the carbon is numbered with substituent that leads to the lowest set of locants (a figure to indicate the position of a functional group within a molecule). Step 3 Conditions Rule 1 When a single ring system is attached to a single chain with a greater number of carbon atoms, it is appropriate to name the compounds as cycloalkylalkane. Example: cyclopentylhexane Rule 2 When more than one ring system is attached to a single chain, it is appropriate to name the compounds as cycloalkylalkane Example: 1,3-dicyclohexylpropane  6.1.2 Nomenclature of alkanes Exercises Name the following compounds according to the IUPAC nomenclature a) c) b) d) Ans: a) 2-cyclopropylpentane b) Isopropylcyclopentane c) 1-ethyl-3-isopropylcyclopentane d) 5-chloro-1-ethyl-2-isopropyl-1-methylcyclohexane  6.1.3 Physical Properties of Alkanes & Cycloalkanes Physical state At room conditions, 25oC and 1 atm pressure, for unbranched alkanes Hydrocarbon Physical state C1 – C4 Gases C5 – C17 Liquids C18 – more Solids Boiling points Depends on the strength of intermolecular forces between molecules C-H bond is a non-polar bond. Intermolecular forces are Van der Waals forces Factors that effect the boiling point 1. Molecular weight / Molar Mass Molecule size bigger, larger surface area in contact, stronger Van der Waals forces Higher heat energy required to overcome the forces, thus higher boiling points. Example: Butane Pentane Hexane CH3CH2CH2CH3 CH3CH2CH2CH2CH3 CH3CH2CH2CH2CH2CH3 BP = 0oC BP = 36oC BP = 79oC 2. Isomeric alkanes – effect of branching chain Molecules with more branches, become more compact, smaller surface area in contact, weaker Van der Waals forces Lower heat energy required to overcome the forces, thus lower boiling points IMPORTANT! Compare effect of branching chain among the isomers only (same molecular weight) Boiling point of branched alkane is higher than straight alkane if number of C is greater Example: Pentane 2-methylbutane 2,2-dimethylpropane CH3CH2CH2CH2CH3 BP = 36oC BP = 10oC BP = 28oC  6.1.3 Physical Properties of Alkanes & Cycloalkanes Cyclic compound The boiling points of cycloalkanes are 10oC to 15oC higher than the corresponding straight chain alkanes Reason: The shape is more compact and flat, intermolecular forces increase due to the larger surface area in contact between molecules Stronger Van der Waals forces: higher heat energy required to overcome the Van der Waals forces, higher boiling point compare to alkanes Example: Cycloalkanes Boiling Points Alkane Boiling Points Cyclobutane BP = 13oC Butane BP = -0.5oC Cyclopentane BP = 49oC Pentane BP = 36.3oC Conclusion: For same molecular weight: Boiling Point: cyclic compound > straight chain > branching chain 6.1.3 Physical Properties of Alkanes More Examples  6.1.3 Physical Properties of Alkanes Refining crude petroleum into usable fuel and other petroleum products. (b) Schematic of a refinery tower. As crude (a) An oil refinery. At an oil refinery, crude petroleum is heated, the lower-boiling, more petroleum is separated into fractions of similar volatile components distill first, followed by boiling point by the process of distillation. fractions of progressively higher boiling point. 6.1.3 Physical Properties of Alkanes & Cycloalkanes Additional Notes  6.1.4 Reaction: Oxidation of Alkanes Since alkanes are the only family of organic molecules that have no functional group, they undergo very few reactions. One reaction that alkanes undergo is combustion. Combustion is an oxidation-reduction reaction Recall that oxidation is the loss of electrons and reduction is the gain of electrons. To determine if an organic compound undergoes oxidation or reduction, we concentrate on the carbon atoms of the starting material and the product, and compare the relative number of C—H and C—Z bonds, where Z = an element more electronegative than carbon (usually O, N, or X).  6.1.4 Reaction: Oxidation of Alkanes Oxidation results in an increase in the number of C—Z bonds; or oxidation results in a decrease in the number of C—H bonds. Reduction results in a decrease in the number of C—Z bonds; or reduction results in an increase in the number of C—H bonds.  6.1.5 Reaction: Combustion of Alkanes Alkanes undergo combustion—that is, they burn in the presence of oxygen to form carbon dioxide and water. This is an example of oxidation. Every C—H and C—C bond in the starting material is converted to a C—O bond in the product.  6.1.6 Reaction Mechanism: Halogenation of alkanes Will be discussed in Chapter 11 (Radical Reactions) ALKANES SCOPE Part 2: Alkenes Name alkenes according to the IUPAC nomenclature for straight chain and branched alkenes Classification and the stability of alkenes Determine physical properties of alkenes Preparation of alkenes: dehydration of alcohols and dehydrohalogenation of alkyl halide Specify types of reactions for alkenes Addition reaction Part 2: Alkenes Learning outcomes At the end of this lecture, students should be able to: Name alkenes according to IUPAC system Determine physical properties of alkenes Specify types of reactions for alkenes  6.2.1 Introduction to Alkenes Structure and Bonding Alkenes are also called olefins. Alkenes contain a carbon—carbon double bond. There are 3 type of alkenes: Terminal alkenes have the double bond at the end of the carbon chain. Internal alkenes have at least one carbon atom bonded to each end of the double bond. Cycloalkenes contain a double bond in a ring.  6.2.1 Introduction to Alkenes Structure and Bonding Properties of C=C double bond Will be covered in Part 6.2.4  6.2.1 Introduction to Alkenes Structure and Bonding Whenever the two groups on each end of a carbon-carbon double bond are different from each other, two diastereomers are possible.  6.2.1 Introduction to Alkenes Degree of Unsaturation Calculating Degrees of Unsaturation An acyclic alkene has the general structural formula CnH2n Alkenes are unsaturated hydrocarbons because they have fewer than the maximum number of hydrogen atoms per carbon. Cycloalkanes also have the general formula CnH2n. Each  bond or ring removes two hydrogen atoms from a molecule, and this introduces one degree of unsaturation.  6.2.1 Introduction to Alkenes Degree of Unsaturation The number of degrees of unsaturation for a given molecular formula can be calculated by comparing the actual number of H atoms in a compound and the maximum number of H atoms possible. This procedure gives the total number of rings and/or  bonds in a molecule.  6.2.1 Introduction to Alkenes Summary General formula: CnH2n, n ≥ 2 Functional group: -C=C- Unsaturated hydrocarbon Each carbon atom with double bond is sp2 hybridized Restricted rotation of carbon-carbon double bond causes cis-trans isomerism  6.2.1 Introduction to Alkenes Interesting alkenes  6.2.1 Introduction to Alkenes Interesting alkenes  6.2.2 Nomenclature of Alkenes Naming alkene using IUPAC system- Step 1 Alkene have the -ene Example: Give the IUPAC name: Step 1: Find the parent carbon chain. Rule: Parent chain must contain both carbon atoms of the double bond.  6.2.2 Nomenclature of Alkenes Naming alkene using IUPAC system- Step 2 Step 2: Number the parent chain. Rule 1: Give the C=C group the lower number Rule 2: Apply all the rules of numbering (as per chapter 6)  6.2.2 Nomenclature of Alkenes Naming alkene using IUPAC system- Step 3 & 4 Step 3: Name the prefix Rule: Apply all the rules of nomenclature of alkanes Step 4: Combine all the name (prefix + parents + suffix)  6.2.2 Nomenclature of Alkenes Naming alkene using IUPAC system- Cycloalkene  6.2.2 Nomenclature of Alkenes Summary IUPAC nomenclature of Alkenes The rules are similar to those for alkanes with an additional consideration for the position of its functional group (double bond) Step 1 Determine the parent name by selecting the longest continuous chain that contains the double bond Change the ending –ane to -ene Step 2 Number the longest continuous chain of alkenes (lowest possible number to the double bond) Step 3 Give the IUPAC nomenclature for alkenes Rule 1: Indicate the position of the substituent by the number of the carbon atoms to which they are attached Rule 2: The ending of the alkenes with more than one double bond should be change from –ene to: - diene – if there are two double bonds - triene – if there are three double bonds Example: 1,3butadiene and 1,3,5-heptatriene Rule 3: For stereoisomeric alkenes, prefix cis and trans are used to distinguished the two stereoisomerism - same side: cis - opposite side: trans Example: cis-1,2-dichloroethane trans-1,2-dichloroethene IUPAC nomenclature of Cycloalkenes 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.  6.2.3 Physical Properties Most alkenes exhibit only weak van der Waals interactions, so their physical properties are similar to alkanes of comparable molecular weight. Alkenes have low melting points and boiling points. Melting and boiling points increase as the number of carbons increases because of increased surface area. Alkenes are soluble in organic solvents and insoluble in water. The C—C single bond between an alkyl group and one of the double bond carbons of an alkene is slightly polar because the sp3 hybridized alkyl carbon donates electron density to the sp2 hybridized alkenyl carbon.  6.2.3 Physical Properties A consequence of this dipole is that cis and trans isomeric alkenes often have somewhat different physical properties. cis-2-Butene has a higher boiling point (4 0C) than trans-2-butene (1 0C).  6.2.4 Classification & Stability of alkenes Classification of alkenes Alkenes are classified according to the number of carbon atoms bonded to the carbons of the double bond. Classifying alkenes by the number of R groups bonded to the double bond  6.2.4 Classification & Stability of alkenes Stability of alkenes  6.2.4 Classification & Stability of alkenes Stability of alkenes Whenever the two groups on each end of a carbon-carbon double bond are different from each other, two diastereomers are possible.  6.2.4 Classification & Stability of alkenes Stability of alkenes Because of restricted rotation, two stereoisomers of 2-butene are possible. cis-2-Butene and trans-2-butene are diastereomers, because they are stereoisomers that are not mirror images of each other.  6.2.4 Classification & Stability of alkenes Stability of alkenes In general, trans alkenes are more stable than cis alkenes because the groups bonded to the double bond carbons are further apart, reducing steric interactions.  6.2.4 Classification & Stability of alkenes Stability of alkenes The stability of an alkene increases as the number of R groups bonded to the double bond carbons increases. The higher the percent s-character, the more readily an atom accepts electron density. Thus, sp2 carbons are more able to accept electron density and sp3 carbons are more able to donate electron density. Consequently, increasing the number of electron donating groups on a carbon atom able to accept electron density makes the alkene more stable.  6.2.4 Classification & Stability of alkenes Stability of alkenes trans-2-Butene (a disubstituted alkene) is more stable than cis-2-butene (another disubstituted alkene), but both are more stable than 1-butene (a monosubstituted alkene).  Reactions of alkenes Preparation of alkenes Dehydration of alcohols Saytzeff’s rule Dehydrohalogenation of alkyl halide Reactions of alkenes (Addition Reaction) Hydrohalogenation Markonikov’s rule 6.2.5 Preparation of Alkenes Will be covered in Chapter 8 & 9  6.2.5 Preparation of Alkenes alkenes can be prepared from alkyl halides and alcohols via elimination reactions. These elimination reactions are stereoselective and regioselective, so the most stable alkene is usually formed as the major product (Saytzeff’s rule)  6.2.6 Introduction to Addition Reactions Syn and anti addition Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides.  6.2.6 Introduction to Addition Reactions Types of addition reactions 5 addition reactions of cyclohexene  6.2.7 Hydrohalogenation—Electrophilic Addition of HX General reaction Two bonds are broken in this reaction—the weak  bond of the alkene and the HX bond—and two new  bonds are formed—one to H and one to X. Recall that the H—X bond is polarized, with a partial positive charge on H. Because the electrophilic H end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions.  6.2.7 Hydrohalogenation—Electrophilic Addition of HX Products of hydrohalogenation To draw the products of an addition reaction: 1. Locate the C-C double bond (C=C) 2. Identify the σ bond of the reagent that breaks- namely the H-X bond in hydrohalogenation 3. Break the π bond of the alkene and the σ bond of the reagent, and form two new σ bonds to the C atoms of the double bond  6.2.7 Hydrohalogenation—Electrophilic Addition of HX Mechanism of hydrohalogenation The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In step 1, the alkene is the Lewis base that donates an electron pair to H—Br, the Lewis acid, In step 2, Br¯ is the Lewis base that donates an electron pair to the carbocation, the Lewis acid. 6.2.7 Hydrohalogenation—Electrophilic Addition of HX Mechanism of hydrohalogenation Step 1: Addition of the electrophile (H+) to the π bond Step 2: Nucleophilic attack if Br- on carbocation. (C-X bond formation)  6.2.7 Hydrohalogenation—Electrophilic Addition of HX Selectivity- Markovnikov’s Rule Markovnikov’s rule states that in the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom—that is, the carbon that has the greater number of H atoms to begin with. With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers, but only one is actually formed: This is a specific example of a general trend called Markovnikov’s rule. ALKENES  SCOPE Part 3: Alkynes Introduction – Structure & Bonding Nomenclature Physical Properties Preparation of Alkynes Introduction to Alkyne Reactions Hydrohalogenation & halogenation reactions Part 3: Alkynes Learning outcomes At the end of this lecture, students should be able to: Name alkynes according to IUPAC system Determine physical properties of alkynes Specify types of reactions for alkynes  6.3.1 Introduction to Alkynes Structure and Bonding Alkynes contain a carbon—carbon triple bond. There are two types of alkynes Terminal alkynes have the triple bond at the end of the carbon chain so that a hydrogen atom is directly bonded to a carbon atom of the triple bond. Internal alkynes have a carbon atom bonded to each carbon atom of the triple bond. An alkyne has the general molecular formula CnH2n-2, giving it four fewer hydrogens than the maximum number possible. The triple bond introduces two degrees of unsaturation. Triple bond consists of 2  bonds and one  bond.  6.3.1 Introduction to Alkynes Extra Information Acetylene (H-CC-H) is a colorless gas that burns in oxygen to form CO2 and H2O. The combustion of acetylene releases more energy per mole of product formed than any other hydrocarbons. It burns with a very hot flame and is an excellent fuel.  6.3.1 Introduction to Alkynes Interesting Alkynes  6.3.2 Nomenclature of Alkynes Naming alkene using IUPAC system Alkynes are named in the same general way that are alkenes are named. In the IUPAC system, change the –ane ending of the parent alkane name to the suffix –yne. Choose the longest continuous chain that contains both atoms of the triple bond and number the chain to give the triple bond the lower number. Compounds with two triple bonds are called diynes, those with three are called triynes and so forth. The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, is more often called acetylene, the common name. The two carbon alkyl group derived from acetylene is called an ethynyl group.  6.3.3 Physical Properties The physical properties of alkynes resemble those of hydrocarbons having similar shape and molecular weight. Alkynes have low melting points and boiling points. Melting point and boiling point increase as the number of carbons increases. Alkynes are soluble in organic solvents and insoluble in water. 6.3.4 Preparation of Alkynes Reactions of alkynes Preparation of alkynes Reactions of alkynes (Addition Reaction) Hydrohalogenation Halogenation  6.3.4 Preparation of Alkynes Recall that alkynes are prepared by elimination reactions. A strong base removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne through two successive E2 elimination reactions.  6.3.5 Introduction to Alkyne Reactions General reaction (addition) Like alkenes, alkynes undergo addition reactions because they contain relatively weak  bonds. Two sequential reactions take place: addition of one equivalent of reagent forms an alkene, which then adds a second equivalent of reagent to yield a product having four new bonds.  6.3.5 Introduction to Alkyne Reactions Types of addition reaction 4 addition reactions of 1-butyne  6.3.6 Hydrohalogenation—Electrophilic Addition of HX General reaction Two equivalents of HX are usually used: addition of one mole forms a vinyl halide, which then reacts with a second mole of HX to form a geminal dihalide.  6.3.6 Hydrohalogenation—Electrophilic Addition of HX Selectivity- Markovnikov’s rule 6.3.6 Hydrohalogenation—Electrophilic Addition of HX Adding one equivalent of HX  6.3.7 Halogenation—Addition of X2 Halogens X2 (X = Cl or Br) add to alkynes just as they do to alkenes. Addition of one mole of X2 forms a trans dihalide, which can then react with a second mole of X2 to yield a tetrahalide. ALKYNES SCOPE Part 4: Oxidation and Reduction of Hydrocarbons Introduction to oxidation and reduction Reducing Agents Reduction of Alkenes and Alkynes Oxidation of Alkenes, Alkynes and Alcohols  Learning outcomes At the end of this lecture, students should be able to: Determine features of oxidation and reduction reactions for organic compounds Determine products of oxidation and reduction reactions  6.4.1 Introduction to oxidation and reduction Oxidation results in an increase in the number ofC—Z bonds (usually C—O bonds) or a decrease in the number of C—H bonds. Reduction results in a decrease in the number of C—Z bonds (usually C—O bonds) or an increase in the number of C—H bonds. 6.4.1 Introduction to oxidation and reduction Sometimes two carbon atoms are involved in a single oxidation or reduction reaction, and the net change in the number of C—H or C—Z bonds at both atoms must be taken into account. o The conversion of an alkyne to an alkene, or an alkene to an alkane are examples of reduction because each process adds two new C—H bonds to the starting material.  6.4.2 Reducing Agents There are three types of reductions differing in how H2 is added. 1. The simplest reducing agent is molecular H2. Reductions using H2 are carried out with a metal catalyst. 2. A second way is to add two protons and two electrons to a substrate—that is, H2 = 2H+ + 2e-. o Reductions of this sort use alkali metals as a source of electrons, and liquid ammonia as a source of protons. o These are called dissolving metal reductions. 3. The third way to add H2 is to add hydride (H¯) and a proton (H+). The most common hydride reducing agents contain a hydrogen atom bonded to boron or aluminum. Simple examples include sodium borohydride (NaBH4) and lithium aluminum hydride (LiAlH4). NaBH4 and LiAlH4 deliver H¯ to the substrate, and then a proton is added from H2O or an alcohol. 6.4.3 Reduction of Alkenes Catalytic Hydrogenation The addition of H2 occurs only in the presence of a metal catalyst, and thus it is called catalytic hydrogenation. The catalyst consists of a metal—usually Pd, Pt, or Ni, adsorbed onto a finely divided inert solid, such as charcoal. H2 adds in a syn fashion. 6.4.3 Reduction of Alkenes Catalytic Hydrogenation The Ho of hydrogenation, also known as the heat of hydrogenation, can be used as a measure of the relative stability of two different alkenes that are hydrogenated to the same alkane. When hydrogenation of two alkenes gives the same alkane, the more stable alkene has the smaller heat of hydrogenation. 6.4.3 Reduction of Alkynes Catalytic Hydrogenation There are three different ways in which H2 can add to the triple bond: 6.4.3 Reduction of Alkynes Catalytic Hydrogenation Alkane formation: 6.4.3 Reduction of Alkynes Product: Cis Alkene Palladium metal is too reactive to allow hydrogenation of an alkyne to stop after one equivalent of H2 adds. To stop at a cis alkene, a less active Pd catalyst is used—Pd adsorbed onto CaCO3 with added lead(II) acetate and quinoline. This is called Lindlar’s catalyst. Compared to Pd metal, the Lindlar catalyst is deactivated or “poisoned”. With the Lindlar catalyst, one equivalent of H2 adds to an alkyne to form the cis product. Reduction of an alkyne to a cis alkene is a stereoselective reaction, bc only one stereoisomer form 6.4.3 Reduction of Alkynes Product: Trans Alkene In a dissolving metal reduction (such as Na in NH3), the elements of H2 are added in an anti fashion to form a trans alkene. Summary of Alkyne Reductions  6.4.4 Oxidation of Alkenes Ozonolysis Oxidative cleavage of an alkene breaks both the  and  bonds of the double bond to form two carbonyl compounds. Cleavage with ozone (O3) is called ozonolysis. 6.4.5 Oxidation of Alkynes Ozonolysis Alkynes undergo oxidative cleavage of the  and both  bonds. Internal alkynes are oxidized to carboxylic acids (RCOOH). Terminal alkynes afford a carboxylic acid and CO2 from the sp hybridized C—H bond.  OXIDATION AND REDUCTION OF HYDROCARBONS The End