3.4 Alkenes Flashcards
What are Alkenes
Alkenes are unsaturated hydrocarbons
Alkenes contain a carbon- carbon double bond somewhere in their structure
What is the General Formula for Alkenes
CnH2n
How do we name alkene positional isomers
Numbers need to be added to the name when positional isomers can occur
What do C=C double bonds consist of?
- C=C double covalent bond consists of one sigma (σ) bond and one pi (π) bond
- π bonds are exposed and have high electron density
- They are therefore vulnerable to attack by species which ‘like’ electrons: these species are called electrophiles
Define an Electrophile
an electron pair acceptor
The double bonds in alkenes are areas with high electron density. This attracts electrophiles and the alkenes undergo addition reactions
Define an addition reaction
A reaction where two molecules react together to produce one
Reaction of bromine with alkenes:
Define a Carbocation
The intermediate formed during electrophilic addition reactions of alkenes, which has a positive charge on the carbon atom
Reaction of hydrogen bromide with alkenes
Why is this carbocation intermediate the most stable?
the methyl groups on either side of the positive carbon are electron releasing and reduce the charge on the ion which stabilises it
In electrophilic addition to alkenes, the major product is formed via the more stable carbocation intermediate
What is the order of stability for carbocations
tertiary > secondary >primary
What are stereoisomers
Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms
When do E-Z Isomers arise?
- There is restricted rotation around the C=C double bond
- There are two different groups/atoms attached both ends of the double bond
What is a priority group?
- The atom with the bigger atomic number is classed as the priority atom
How do we name E-Z Isomers?
- First determine the priority groups on both sides of the double bond
Reaction of sulfuric acid with alkenes
Define Hydrolysis
a reaction where the molecule is split by the addition of water
What are the essential conditions for the hydration of alkenes to form alcohols
Electrophilic addition mechanism summary
- Electrophile attracted to double bond
- Electrophiles are positively charged so accept electron pair from double bond
- A positive ion (carbocation) is formed
- A negatively charged ion forms a bond with the carbocation
Electrophilic addition bromine summary
Electrophilic addition HBr summary
Electrophilic addition H2SO4 summary
What is a major product?
Most stable. On the secondary or tertiary carbocation. You need to put the electrophile on the carbon closest to the end
What is a minor product?
less stable, on primary carbocation
Test for double bond
Complete the elimination mechanism by drawing appropriate curly arrows (3)
Draw structures for the E and Z stereoisomers of hex-3-ene (2)
State the meaning of the term stereoisomers (2)
- (Compounds / molecules with) the same structural formula
- with atoms/bonds/groups arranged differently in space
State the type of structural isomerism shown by but-1-ene and but-2-ene (1)
Position(al) (isomerism)
The first stage in this conversion involves the reaction of hydrogen bromide with but-1-ene.
Outline a mechanism for this reaction (4)
The second stage is to convert 2-bromobutane into but-2-ene
Outline a mechanism for this reaction (3)
Name and outline a mechanism for the following conversion (5)
State what is meant by the term unsaturated as applied to an alkene (1)
Contains a C=C OR a double bond
What feature of the double bond prevents isomer 1 from changing into isomer 2? (EZ) (1)
restricted rotation OR no rotation OR cannot rotate
Role of OH in substitution
Nucleophile
Role of OH in elimination
Base
Name and outline the mechanism for the reaction of cyclohexene with Br2 (5)
Explain why there is an attraction between a C=C double bond and Br2 (3)
- C=C electron rich / area of high electron density
- Br-Br becomes polarised
- δ+ Br attracted to C=C
Explain why more of isomer E than isomer F is formed in this reaction (2)
- M1 idea that E is formed from/via more stable carbocation
- idea that 2y carbocation is more stable than 1y carbocation
2-Methyl but-2-ene reacts with concentrated sulfuric acid to form two different products.
(a) Outline a mechanism for this reaction to show the formation of the major product (4)
Explain why the two products are formed in different amounts (2)
- The major product is formed via a tertiary carbocation intermediate and the minor product is formed via a secondary carbocation intermediate
- The tertiary carbocation is more stable than the secondary carbocation
State what you would observe if bromine water was added to
poly(chloroethene). Explain this observation (2)
- no reaction / yellow-orange
- polymer is saturated / does not contain double bond(s)
- M1 conc H2SO4 or conc H3PO4
- M2 heat / 170°C
