(U2) Alkenes

Question 1

What is an unsaturated hydrocarbon?

Answer 1

Compound containing at least 1 C=C or C≡C bond

Question 2

What is a sigma bond? (2)

Answer 2

• A covalent bond
• formed by the linear overlap of atomic orbitals (2s, 1s and 1p or 2p)

Question 3

What is a Pi bond? (2)

Answer 3

• A covalent bond
• formed by the sideways overlap of p orbitals

Question 4

What is bond length?

Answer 4

The distance between the nuclei of two covalently bonded atoms

Question 5

What is hydrogenation?

Answer 5

Addition of a hydrogen molecule across a C=C bond

Question 6

What is an electrophile

Answer 6

An ion or molecule that attacks regions of high electron density

Question 7

What is a primary carbocation?

Answer 7

A carbocation which has 1 carbon atom directly bonded to the positively charged carbon

Question 8

What is a secondary carbocation?

Answer 8

A carbocation which has 2 carbon atoms directly bonded to the positively charged carbon

Question 9

What is a tertiary carbocation?

Answer 9

A carbocation which has 3 carbon atoms directly bonded to the positively charged carbon

Question 10

What is polymerisation?

Answer 10

Joining together of many monomers to form a large polymer molecule

Question 11

What are monomers?

Answer 11

Many small molecules which join together to form a polymer

Question 12

What is a polymer?

Answer 12

A large molecule formed when monomers join together

Question 13

What is the general formula of alkenes?

Answer 13

C_nH_2n

Question 14

What are the physical properties of alkenes? (3)

Answer 14

• boiling point increases with length of carbon chain - more VdV
• alkenes have 2 fewer electrons than their corresponding alkane, therefore alkanes have a marginally higher b.p. - more VdV
• insoluble in water, but soluble in organic solvents

Question 15

Comparatively, which bond is stronger; a pi bond or a sigma bond?

Answer 15

Sigma bond - as a result has higher bond energy.

(Pi bonds break easily)

Question 16

What is formed by the hydrogenation of alkenes?

Under what conditions does the reaction occur?

Answer 16

• Alkanes
• high temp and pressure (150-300ºC and 4 atm)

Question 17

What is the catalyst for the hydrogenation of alkenes?

Outline the process (5) of hydrogenation of ethene

Answer 17

1. Finely divided nickel
   2.

• ethene adsorbs onto the catalyst
• the hydrogen molecule also adsorbs and breaks into atoms
• the hydrogen atom combines with ethene to form an ethyl radical
• a second hydrogen atom combines with the ethyl radical
• ethane desorbs from he catalyst

Question 18

What is the test for alkenes?

Answer 18

Bromine water turns from brown to colourless

Question 19

Why are alkenes more reactive than alkanes?

(3)

Answer 19

The C=C bond possesses a weak, electron rich π bond:

• π bond has lower dissociation energy than a sigma bond, meaning it is easier to break; alkanes only have strong sigma bonds
• due to the π bond, there are regions of high electron density above and below the plane (where the π electrons are delocalised)
• this makes them susceptible to electrophillic attack

Question 20

What type of mechanism is the addition of hydrogen halides to an alkene?

Answer 20

Electrophilic addition

Question 21

Outline the process of addition of bromine to ethene (4)

Answer 21

• bromine molecule approaches the ethene and becomes polarised by the -ve charge of the C=C
• a loose association forms between the ethene and bromine molecules
• a C-Br bond forms along with a carbocation and a bromine ion
• the carbocation is unstable and reacts with the bromine ion

Product = 1,2-dibromoethane

Question 22

Outline the process of addition of HBr to ethene (3)

Answer 22

(Solutions of HBr = aqueous)

• H⁺ ions act as electrophiles and become attack a region of high electron density around the π bond of the C=C
• a C-H bond forms along with an intermediate carbocation and a bromide ion
• the carbocation is unstable and reacts with the bromide ion

Product = bromoethane

Question 23

What type of mechanism is the addition of halogens to an alkene?

Answer 23

Addition

Question 24

Why is the major product of the addition of dilute HCl to propene 2-bromopropane rather than 1-bromopropane?

Answer 24

• a secondary carbocation is more stable than a primary one
• groups tend to push electrons toward the carbocation, stabilising it (positive inductive effect)
• the secondary carbocation has 2 alkyl groups stabilising, where the primary only has 1

Question 25

Describe how to write the process of addition of Iodine to ethene using curly arrow notation

(4)

Answer 25

• arrow from = to ∂+ iodine atom
• arrow from bond line (-) between iodine atoms to the ∂- iodine atom
• arrow from :I^- to C⁺ on the carbocation
• product

Question 26

What is the name given to the process of converting alkanes to alkenes?

Answer 26

Elimination (dehydrogenation)

Question 27

Why are secondary carbocations more stable than primary ones?

Why does this create minor and major products in reactions?

Answer 27

Central C⁺ is bonded to more alkyl groups

Question 28

Describe how to write the process of addition of HI to ethene using curly arrow notation

(4)

Answer 28

• arrow from = to ∂+ hydrogen atom
• arrow from bond line (-) between iodine atoms to the ∂- iodine atom
• arrow from :I^- to C⁺ on the carbocation
• product