Chapter 6 Unsaturated Hydrocarbons

Naming of alkene, diene, cycloalkene, and cyclodiene cis/trans isomers E/Z isomers

Alkenes

• sometimes called olefins

• have C-C double bonds as functional group

• higher; polarizable

Physical Properties

• alkenes have slightly _________ dipole moments compared to alkanes due to the loosely held pi electrons (easily ____________)

• physical properties of alkenes are very similar to those of alkanes

Systematic Names of Linear Alkenes

• names end in –ene

• except for ethene and propene, locants are used to indicate the position of the carbon to carbon double bond

Systematic Names of Branched Alkenes

• the longest chain that contains both carbon atoms of the double bond is assigned as parent chain

• number the parent chain to give the double bond the lower number

Systematic Names of Alkadienes

• if two carbon to carbon double bonds are present, indicate the position of each and use the suffix -diene

Systematic Names of Cycloalkenes

• the double bond is located between C1 and C2

• the ring is numbered clockwise or counterclockwise to give the first substituent the lower number

Systematic Names of Cycloalkadienes

• the lowest possible set of locants are assigned to the two carbon to carbon double bonds, with one of the double bonds located between C1 and C2

trans - farther

cis - nearer

Cis and trans isomerism in Alkenes

• the C-C double bond is not capable of free rotation

• the energy required to break the π bond in ethene is approximately 268 kJ/mol

Between the two which C1 and C4 are farther from each other and which is nearer?

its as if you just flipped the entire molecule

• to rotate you’d have to break the double bond into a single bond, rotate, then reform the double bond.

Why are they identical or can’t exist as cis-trans isomers?

1. cannot exist as cis-trans isomer

2. cannot exist aas cis-trans isomers because the two CH3 attached ti the 2nd carbon are the same.

3. can exist, drawn below is the trans (top) and cis (bottom) isomers.

4. same reason as number 2.

5. can exist, cis and trans drawn below.

• 

• 

3. C is double bonded to O so imagine 2 O’s and the O is double bonded to the C so imagine 2 C’s.

Alkene Stability

Stereochemistry - trans alkenes are generally more stable than cis alkenes because the groups bonded to the double bond carbons are farther apart, reducing steric interactions

which is more stable trans alkenes or cis alkenes? Why?

Number of substituents across the double bond

• alkene stability increases as the number of alkyl groups bonded to the double bond carbons increases

Number of substituents across the double bond

• alkene stability _____________ as the number of alkyl groups bonded to the double bond carbons increases

Hyperconjugation

___________________ - Stabilizing interaction between the C=C π bond and adjacent C-H o bond on substituents

heat of hydrogenation

Alkene Stability - _____________ can be used to measure the relative stability of two different alkenes that are hydrogenated to the same alkane

smaller

More substituted, more stable

When hydrogenation of two alkenes gives the same alkane, the more stable alkene has the __________ heat of hydrogenation.

Dehydrohalogenation

_________________ of an alkyl halide - usually occurs by reaction of an alkyl halide with strong base

Dehydration

_______________ of alcohol - often carried out by treatment of an alcohol with a strong acid

Electrophilic addition; Addition of halogens

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: Br₂ or Cl₂ in organic solvent, CCl₄ or CH₂Cl₂

Electrophilic addition; Addition of halogens

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

positive visual result: disappearance of redorange bromine color

Electrophilic addition; Addition of halogens

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

stereochemistry: anti-addition (two parts of the reagent are added from opposite sides of the double bond)

Electrophilic addition; Addition of HX

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: HCl, HBr or HI

Markovnikov Rule

the rule that states in the addition of HX to an unsymmetrical alkene, the H atom bonds to the less substituted carbon atom

• In an addition of HX to an alkene, the more highly substituted carbocation is formed as the intermediate rather than the less substituted one.

Inductive Effect

Two concepts to explain carbocation stability:

________________

• shifting of electrons in a sigma bond in response to the electronegativity of nearby atoms

Hyperconjugation

Two concepts to explain carbocation stability:

__________________

• stabilizing interaction between a vacant p orbital and neighboring sigma bond electrons

• Positive charge is spread over a greater volume of space

Electrophilic addition; Halohydrin formation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: Br₂ in H₂O

Electrophilic addition; Halohydrin formation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

• the solvent, water, will compete with bromide as nucleophile

• stereochemistry: anti-addition

• regioselectivity: Markovnikov rule is followed

Electrophilic addition; Reaction with cold, concentrated H₂SO₄

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: cold, conc. H₂SO₄

Electrophilic addition; Hydration

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: dilute H₂SO₄

In electrophilic addition reactions, carbocations may rearrange forming ‘unexpected’ products. What are these changes called?

Given what are the minor and major products?

Free Radical Addition; Addition of HBr with peroxide

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

• reagent: HBr with peroxide

• regioselectivity: anti-Markovnikov (the H atom bonds to the more substituted carbon atom)

Free Radical Addition; Polymerization of alkenes

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: peroxide as initiator

Oxidation; Ozonolysis

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: (1) O₃ ; (2) Zn, H₃O+

aldehydes; ketones

Products of ozonolysis of alkenes are __________ and/or ___________ depending on the structure.

Oxidation: Reaction with acidic KMnO₄

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: KMnO₄ , H₃O+

What are the respective products of each reactant in Oxidation: Reaction with acidic KMnO₄?

Oxidation with KMnO₄

reagent: KMnO₄

What is the positive visual result of this process.

Oxidation; Hydroxylation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: cold, dilute, neutral KMnO₄
stereochemistry: syn addition

What is the product of this Oxidation; Hydroxylation reaction?

Oxidation; Hydroxylation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

Oxidation; Epoxidation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

reagent: m-chloroperoxybenzoic acid

Oxidation; Epoxidation

State which type of Reactions of Alkenes (Electrophilic addition, Free Radical Addition, Oxidation) and which specific reaction under each category.

What is the product of this Oxidation; Epoxidation reaction?

conjugated

Alkadienes - have two sets of C-C double bonds

cumulated

Alkadienes - have two sets of C-C double bonds

isolated

Alkadienes - have two sets of C-C double bonds

Conjugation

1,3-diene

_______________ - occurs when p orbitals can overlap on three or more adjacent atoms

_____________ - contains two carbon-carbon double bonds joined by a single σ bond

Allylic carbocation

___________________

- a conjugated system

- the p orbitals of the double bond carbons each contain an electron, while the p orbital of the carbocation is empty

What are the products?

Alkynes

____________ - have C-C triple bonds as functional group

• ethyne; propyne

  

Systematic Names of Linear Alkynes

• names end in –yne

• except for _________ and ___________, locants are used to indicate the position of the carbon to carbon triple bond

Internal Alkynes