40 Questions
What is another name for alkenes?
olefins
What type of bond is present in the double bond of an alkene?
both a σ bond and a π bond
What is the characteristic of a terminal alkene?
the double bond is at the end of the carbon chain
What type of alkene has at least one carbon atom bonded to each end of the double bond?
internal alkene
What type of alkene has a double bond in a ring?
cycloalkene
What is the term for the energy required to break a bond?
bond dissociation energy
What is the reason for addition reactions being exothermic?
The two σ bonds formed in the product are stronger than the σ and π bonds broken in the reactants.
What is the bond dissociation energy of?
a C-C bond in ethane
In the mechanism of electrophilic addition, what is the role of the alkene?
It acts as a Lewis base and donates an electron pair.
What is the type of bond that is not mentioned in the context of alkenes?
ionic bond
What is the characteristic of the energy diagram for electrophilic addition?
Each step has its own energy barrier with a transition state energy maximum.
Why is step 1 the rate-determining step in electrophilic addition?
Because it has a higher energy transition state.
What is the sign of ΔH° for step 1 of electrophilic addition?
Positive, because more bonds are broken than formed.
What is Markovnikov's rule stating in the addition of HX to an unsymmetrical alkene?
The H atom adds to the less substituted carbon atom.
What is the result of the addition of HX to an unsymmetrical alkene?
Two constitutional isomers are formed, but only one is actually formed.
What is the value of ΔH° for the addition of HBr to ethylene?
-14 kcal/mol
What determines whether a triacylglycerol is a fat or an oil?
The number of double bonds in the fatty acids
What is broken in the characteristic reaction of alkenes in addition?
The π bond
Which of the following is a characteristic of saturated fats?
They are typically obtained from animal sources
What is the major product formed in the preparation of alkenes from alkyl halides?
The most stable alkene (Zaitsev product)
What is the result of the addition reaction of alkenes?
The formation of two new σ bonds
Where is the red electron-rich region of the π bond located in the electrostatic potential plot of ethylene?
Above and below the plane of the molecule
What is the source of coconut oil?
Vegetable sources
What is the typical physical state of oils at room temperature?
Liquid
What is the effect of each π bond or ring on the number of hydrogen atoms in a molecule?
It removes two hydrogen atoms
How do you calculate the number of degrees of unsaturation for a given molecular formula?
By comparing the actual number of H atoms to the maximum number of H atoms possible
What is the effect of a nitrogen atom on the number of hydrogen atoms in a molecule?
It subtracts one hydrogen atom
How are compounds with both a double bond and a hydroxy group named?
As alkenols with the chain or ring numbered to give the OH group the lower number
What prefix is used to indicate that the two alkyl groups are on the same side of the double bond in an alkene?
Cis
How are compounds with two double bonds named?
As dienes by changing the '-ane' ending of the parent alkane to the suffix '-diene'
What is the purpose of calculating the number of degrees of unsaturation?
To determine the number of π bonds and rings in a molecule
How are halogens and hydrogen atoms treated in calculating the number of degrees of unsaturation?
Halogens are treated as equivalent to hydrogen atoms
What is the result of nucleophilic attack of Cl¯ on the trigonal planar carbocation?
A racemic mixture of two enantiomers
What is the product of the addition of water to an alkene?
An alcohol
What is the role of the electron-rich double bond in halogenation?
It induces a dipole in the approaching halogen molecule
What is the outcome of the addition of CH3OH to 2-methylpropene?
Formation of an ether
Why do halogens add to π bonds?
Because they are polarizable
What is a characteristic of halogenation that distinguishes it from hydrohalogenation or hydration?
No rearrangements occur and only anti addition of X2 is observed
What is the electrophilic species in the halogenation mechanism?
The electron-deficient halogen atom
What type of addition is observed in halogenation?
Anti addition
Study Notes
Alkene Structure
- Alkenes are also called olefins.
- Alkenes contain a carbon-carbon double bond.
- 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.
Alkene Hybridization
- The double bond consists of a π bond and a σ bond.
- Each π bond or ring removes two hydrogen atoms from a molecule, introducing one degree of unsaturation.
Bond Dissociation Energy
- Bond dissociation energies of C-C bonds in ethane are important to consider.
Naming Alkenes and Alkenols
- Compounds with a double bond and a hydroxy group are named as alkenols.
- The chain (or ring) is numbered to give the OH group the lower number.
Naming Polyenes and Cyclic Alkenes
- Compounds with two double bonds are named as dienes.
- Compounds with three double bonds are named as trienes.
Properties of Fatty Acids
- Fats are derived from fatty acids having few or no double bonds.
- Oils are derived from fatty acids having a larger number of double bonds.
- Saturated fats are typically obtained from animal sources.
- Unsaturated oils are common in vegetable sources.
Preparation of Alkenes
- Alkenes can be prepared from alkyl halides, tosylates, and alcohols via elimination reactions.
- The most stable alkene (Zaitsev product) is usually formed as the major product.
Addition Reactions
- The characteristic reaction of alkenes is addition, where the π bond is broken and two new σ bonds are formed.
- The red electron-rich region of the π bond is located above and below the plane of the molecule.
Heat of Formation for Electrophilic Addition
- Addition reactions are exothermic because the two σ bonds formed in the product are stronger than the σ and π bonds broken in the reactants.
Mechanism of Electrophilic Addition
- The mechanism consists of two successive Lewis acid-base reactions.
- Step 1: the alkene is the Lewis base that donates an electron pair to H-Br, the Lewis acid.
- Step 2: Br¯ is the Lewis base that donates an electron pair to the carbocation, the Lewis acid.
Energy Diagram for Electrophilic Addition
- Each step has its own energy barrier with a transition state energy maximum.
- With an unsymmetrical alkene, HX can add to the double bond to give two constitutional isomers.
Markovnikov’s Rule
- In the addition of HX to an unsymmetrical alkene, the H atom adds to the less substituted carbon atom.
Stereochemistry of Nucleophilic Attack
- Nucleophilic attack of Cl¯ on the trigonal planar carbocation occurs from two different directions, forming two products, A and B, having a new stereogenic center.
- A and B are enantiomers, and a racemic mixture is formed.
Hydration
- Hydration is the addition of water to an alkene to form an alcohol.
Mechanism of Hydration
- Hydration follows the same mechanism as electrophilic addition.
Electrophilic Addition of Alcohols
- Alcohols add to alkenes, forming ethers by the same mechanism.
Halogenation
- Halogenation is the addition of X2 (X = Cl or Br) to an alkene to form a vicinal dihalide.
- Halogens add to π bonds because halogens are polarizable.
Halogenation Mechanism
- The electrophilic halogen atom is attracted to the nucleophilic double bond, making addition possible.
- Two facts demonstrate that halogenation follows a different mechanism from that of hydrohalogenation or hydration: • No rearrangements occur. • Only anti addition of X2 is observed.
Learn about the structure of alkenes, also known as olefins, including their carbon-carbon double bonds and terminal alkenes.
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