Lecture 3 - Alkene & Alkyne Lecture Notes PDF

Chapter 3:Alkene & Alkyne Dr. Roula Bayram  Alkene  Alkenes, also called olefins, are hydrocarbons that contain a carbon-carbon double bond.  General molecular formula CnH2n  They are unsaturated hydrocarbons  The class suffix of alkenes is “en...

Chapter 3:Alkene & Alkyne Dr. Roula Bayram  Alkene  Alkenes, also called olefins, are hydrocarbons that contain a carbon-carbon double bond.  General molecular formula CnH2n  They are unsaturated hydrocarbons  The class suffix of alkenes is “ene” chapter3: Alkenes & Alkynes 2  Alkene Nomenclature Step 1: Name the parent hydrocarbon. Find the longest carbon chain that contains the double bond, and name the compound using the suffi x -ene in place of -ane. Step 2: Number the carbon atoms in the chain  Number carbons in chain so that double bond carbons have lowest possible numbers  Indicate the position of the double bond with a number and change the ending to “ene”  If there is more than one double bond use “diene”, “triene”.. Step3: Write the full name. chapter3: Alkenes & Alkynes 3 Step4:  When there are both a double bond and a substituent( alkyl or halogens), the double bond gets the lowest number.  The double bond takes precedence over alkyl groups and halogens when the chain is numbered.  But Hydroxyl groups take precedence over the double bond when the chain is numbered. CH3 CH2 CH CH CH2OH 4 3 2 1 2-methyl-3-buten-1-ol chapter3: Alkenes & Alkynes 4 Step5: The substituents are listed alphabetically: Step6: Name with the lowest functional group number and then the lowest substituent numbers: Chapter 3. Alkenes & Alkynes 5 Step7: In a cyclic alkene, the double bond is given the number 1, but the -1-is left out of the name. Replace the -ane ending of the cycloalkane having the same number of carbons by -ene. Step8: The alkene substituent is given the lower number. Number through the double bond in the direction that gives the lower number to the first-appearing substituent. chapter3: Alkenes & Alkynes 6 Special Nomenclutuer chapter3: Alkenes & Alkynes 7  Exercise 1- 2- 3- Solution: 1 2-Chloro-4-Methyl-3-hexene 2 2-Bromo-5-chloro-3-hexene 3 2-Ethyl-1-pentene chapter3: Alkenes & Alkynes 8  The Electronic Structure of Alkenes  The two sp2 hybridized carbon atoms then make a sigma (σ) bond by a linear overlap and one π bond by a side-by-side overlap of the two 2p orbitals on each carbon.  Ethylene can be used as an example to illustrate the structure and geometry of a typical double bond.  As a short summary for the key parameters about double bonds, remember that:  All the atoms on the double bond are in one plane.  The angle between the atoms is 120o. chapter3: Alkenes & Alkynes 9  Cis-Trans Isomers of Alkenes  Remember that alkanes and cycloalkanes that they were able to adopt different conformations via the free rotation about sigma bonds. For example, this flexibility allows for the ring flip  Now, unlike the single bonds, the C=C double bond is locked as there is no rotation about the bond without breaking it. chapter3: Alkenes & Alkynes 10  Because of this we have the cis and trans isomerism of alkenes.  if the two identical groups (alkyl or hydrogen) on both carbon atoms of the double bond are on the same side of the double bond, then it is cis and if they are on opposite sides of the double bond, then we have a trans isomer  The cis isomer is less stable because of steric interactions. chapter3: Alkenes & Alkynes 11 chapter3: Alkenes & Alkynes 12  Example 1- Draw the cis and trans isomers of 5-chloropent-2-ene. 2- Test yourself: https://docbrown.info/page06/PRalkenes/alkeneqcomb.htm chapter3: Alkenes & Alkynes 13  Naming stereoisomerism of Alkene by the E,Z Designation  According to the E,Z system, a set of sequence rules is used to rank the two substituent groups on each double-bond carbon.  The substituents are ranked in order of decreasing atomic number. E : higher ranked substituents on opposite sides for the German entgegen, meaning “opposite.” Z : higher ranked substituents on same side for the German zusammen, meaning “together chapter3: Alkenes & Alkynes 14  Cahn–Ingold–Prelog rules (CIP)System  Rule1: Higher atomic number outranks lower atomic number. (Atoms with higher atomic numbers have higher priority) chapter3: Alkenes & Alkynes 15  Rule2: If a decision can’t be reached by ranking the first atoms in the substituents, look at the second, third, or fourth atoms away from the double-bond carbons until the first difference is found. Rule 3: Multiple-bonded atoms are equivalent to the same number of single- bonded atoms. For example, an aldehyde substituent ( CHO), which has a carbon atom doubly bonded to one oxygen, is equivalent to asubstituent having a carbon atom singly bonded to two oxygens. chapter3: Alkenes & Alkynes 16  By applying the sequence rules, we can assign the stereochemistry shown in the following examples.  Example: chapter3: Alkenes & Alkynes 17  Exercise Name the following compound and then Assign E or Z stereochemistry to the following compounds: 1 2 3 Nomenclature -E or Z stereochemistry 1 2 3 chapter3: Alkenes & Alkynes 18  polyenes  Systems with more than one C=C can be referred to as "polyenes"  The simplest types of polyenes are those in which there are two double bonds, the "dienes".  Dienes are classified according to the relative arrangement of the double bonds into:  Conjugated dienes  Isolated dienes  Cumulated dienes H2C C CH2 Note; Conjugated dienes > Isolated dienes > Cumulated dienes Decrease in stability chapter3: Alkenes & Alkynes 19  Alkene Stability  The more substitute alkene the more stable alkene  The more α-Hydrogen is the more stable alkene Note:  The alpha carbon (Cα) in organic molecule refers to the first carbon atom that attaches to a function group. The second carbon atom is called the beta carbon (Cβ)  A hydrogen atom attached to an alpha carbon atom is called an alpha hydrogen atom, a hydrogen atom on the beta-carbon atom is a beta hydrogen atom, and so on. chapter3: Alkenes & Alkynes 20 chapter3: Alkenes & Alkynes 21  Synthesis of Alkene  Alkenes can be prepared from alkyl halides and from alcohols by elimination reactions 1- Elimination of HX from alkyl halide (dehydrohalogenation) using Alcoholic KOH: chapter3: Alkenes & Alkynes 22 2-Elimination of water (H-OH) from an alcohol (dehydration) chapter3: Alkenes & Alkynes 23  Saytzeff Rule  During the elimination reaction proton is removed from the carbon atom having less number of substituents(fewer hydrogen). The corresponding olefin is known as the Saytzeff's product. chapter3: Alkenes & Alkynes 24  Alkene Reaction Electrophilic Addition Step 1: Pi electrons attack the electrophile. E + C C + E C C + Step 2: Nucleophile attacks the carbocation E E Nuc _ C C + + Nuc: C C Note that: Electrophile : poor of electron Nucleophile : electron rich Carbocations are excellent electrophiles and react readily with nucleophiles. R X R X The general stability order of carbocations is: (most stable) 3o> 2o> 1o> methyl (least stable) chapter3: Alkenes & Alkynes 25 Electrophilic Addition chapter3: Alkenes & Alkynes 26 Protonation of double bond yields the most stable carbocation. The proton is added to the carbon with less substituents the positive charge goes to the carbon that was not protonated. chapter3: Alkenes & Alkynes 27  1- Addition of HX (hydrohalogenation) Markovnikov’s Rule  In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents.  When both double-bond carbon atoms have the same degree of substitution, a mixture of addition products results. chapter3: Alkenes & Alkynes 28 chapter3: Alkenes & Alkynes 29 The overall reaction of HX Addition Step 1 Step 2 CH3 CH3 CH3 C CH CH3 CH3 C CH CH3 + H Br H _ Br Chapter 3. Alkenes & Alkynes 30 chapter3: Alkenes & Alkynes 31  Example What product would you expect from the reaction of HCl with 1-ethylcyclopentene? Solution: chapter3: Alkenes & Alkynes 32  Addition of HBr In absence of peroxide In presence of peroxide H2O2 Anti-Markonvikov's product 1.Heterolytic fission of HBr bond 1.Homolytic fission of HBr bond 2. The electrophile is the proton 2. The electrophile is the bromine free radical 3. Markovnikov's product 3. Anti-Markonvikov’s product 4. IUPAC name of the product 4. IUPAC name of the product 2-Bromo-2-methylbutane 2-Bromo-3-methylbutane Chapter 3. Alkenes & Alkynes 33  2- Addition of Water : Acid- Catalyzed Hydration  Hydration is the addition of water to alkene will take place in the presence of concentrated sulfuric acid to give an alcohol according to Markovnikov’s rule. + H OH H C C + H2O C C alkene alcohol chapter3: Alkenes & Alkynes 34  Example  Complete the following reaction? 1 2  Solution: 1 2 chapter3: Alkenes & Alkynes 35  3- Addition of Halogen (Br, Cl) Halogenation  Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide.  Example: chapter3: Alkenes & Alkynes 36  4- Formation of Halohydrin  The reaction of an alkene with a halogen X2 and H2O forms a halohydrin by addition of the elements of X and OH to the double bond. chapter3: Alkenes & Alkynes 37  5- Addition of Hydrogen (Hydrogenation)  Hydrogenation is a reaction of an alkene with hydrogen by used transition metal catalyst include platinum, palladium, and nickel because the conversion of an alkene to an alkane involve reduction of hydrogen in the presence of a catalyst. The process is called catalytic reduction or catalytic hydrogenation. chapter3: Alkenes & Alkynes 38 Alkynes chapter3: Alkenes & Alkynes 39  Introduction  Alkynes contain a carbon—carbon triple bond.  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 possible for the number of carbons present. Recall that the triple bond consists of 2  bonds and 1  bond. Each carbon is sp hybridized with a linear geometry and bond angles of 180o. chapter3: Alkenes & Alkynes 40  IUPAC Nomenclature  Alkynes are named in the same general way that 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 named as diynes, those with three are named as triynes and so forth.  Compounds with both a double and triple bond are named as enynes. The chain is numbered to give the first site of unsaturation (either C=C or CC) the lower number.  The simplest alkyne, H-CC-H, named in the IUPAC system as ethyne, is more often called acetylene, its common name.  The two-carbon alkyl group derived from acetylene is called an ethynyl group. chapter3: Alkenes & Alkynes 41 common nomenclature chapter3: Alkenes & Alkynes 42  Example chapter3: Alkenes & Alkynes 43  Synthesis of Alkyne  alkynes are prepared by elimination reactions. A strong base like KOH or NaNH2  removes two equivalents of HX from a vicinal or geminal dihalide to yield an alkyne through two successive E2 elimination reactions. chapter3: Alkenes & Alkynes 44  Alkyne Reaction  Like alkenes, alkynes undergo addition reactions because they contain relatively weak  bonds.  Two sequential reactions can take place: addition of one equivalent of reagent forms an alkene, which can then add a second equivalent of reagent to yield a product having four new bonds. chapter3: Alkenes & Alkynes 45 chapter3: Alkenes & Alkynes 46  Hydrohalogenation—Electrophilic Addition of HX  Alkynes undergo hydrohalogenation, i.e. the, addition of hydrogen halides, HX (X = Cl, Br, I)  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. Chapter 3. Alkenes & Alkynes 47 Chapter 3. Alkenes & Alkynes 48 Halogenation ( addition of halogen Cl2 or Br2 chapter3: Alkenes & Alkynes 49  Hydration—Electrophilic Addition of Water  In the presence of strong acid and Hg2+ catalyst, the elements of H2O add to the triple bond, but the initial addition product, an enol, is unstable and rearranges to a product containing a carbonyl group.  The addition of water is according to Markovnikov’s rule Example: chapter3: Alkenes & Alkynes 50  Keto-enol Tautomerism  Isomeric compounds that can rapidly interconvert by the movement of a proton are called tautomers and the phenomenon is called tautomerism  Enols rearrange to the isomeric ketones by the rapid transfer of a proton from the hydroxyl to the alkene carbon.  The keto form is usually so stable, compared to the enol, that only the keto form can be observed. chapter3: Alkenes & Alkynes 51  Hydrogenation :Addition of H2 (Reduction) Addition of H2over a metal catalyst (such as palladium on carbon, Pd/C) converts alkynes to alkanes (complete reduction) chapter3: Alkenes & Alkynes 52  Incomplete Reduction: Conversion of Alkynes to Cis Alkenes Addition of H2using the Lindlar catalyst produces a cis alkene. Lindlar catalyst is a heterogeneous catalyst that consists of Palladium deposited on Calcium Carbonate which is then poisoned with lead acetate and Quinoline. It is used for the Hydrogenation of Alkynes to Alkenes without further reduction into alkanes. chapter3: Alkenes & Alkynes 53  Incomplete Reduction: Conversion of Alkynes to Trans-Alkenes  Addition of H2using the Sodium or lithium catalyst in presence of NH3 produces a Trans alkene. chapter3: Alkenes & Alkynes 54  Exercise Find the product of the following reaction? Solution: chapter3: Alkenes & Alkynes 55 The End chapter3: Alkenes & Alkynes 56