Alkanes And Alkenes And Halogenoalkanes

Question 1

Why E-Z is possible?

Answer 1

1. It has carbon carbon double bond
   It restricts rotation about the double bond
2. Each doubly bonded carbon is bonded totwo different atoms / groups

Question 2

How can C10 H22 be formed from free radical substitution of pentane

Answer 2

1. One intermediate is C5H11 radical

2. Two of them combine to form C10 H22

Question 3

What is homolytic bond fission?

Answer 3

A covalent bond breaks and each atom receives an electron

Question 4

What is heterolytic bond fission?

Answer 4

A process of bond breaking where the two electrons

(of the covalent bond) go to one of the two atoms in the bond

Question 5

Describe bonding between carbon atoms in ethene. (2 marks)

Answer 5

1. Number of sigma bonds present
   1 sigma bond formed by overlap of s-orbitals
2. Number of pi bond present
   1 pi bond formed by sideways overlap of p-orbitals

Question 6

What is polymerisation?

Answer 6

Joining of many small molecules to make a large molecule

Question 7

Why is a reaction described as an addition reaction?

Answer 7

(Bromine) is added across the double bond

Question 8

Describe bonding in propane and explain its reactivity (6 marks)

Answer 8

Alkenes contain p and sigma bonds
Sigma bonds are formed from s-s orbital overlap / end-on orbital overlap
Pi bonds are formed from sideways overlap of p orbitals / overlap above and below plane
The pi bond gives a region of high electron density
This is susceptible to electrophilic attack/ attack by an electron
deficient species
This attack leads to addition reactions

Question 9

Reaction between methane and chlorine does not produce a pure sample. Why?

Answer 9

Reaction produces a mixture of halogenocompounds/ more than one halogen can be substituted / ethane

Question 10

2-bromobutane reacts to give alkene under what conditions? Name the two products which are structural isomers of each other.

Answer 10

Alcoholic NaOH

But-1-ene and But-2-ene

Question 11

Explain the reactivity difference between alkanes and alkenes. (7 marks)

Answer 11

• alkenes have double bonds and are much more reactive
• due to high electron density of π bond
• caused by p-p sideways overlap
• alkanes are saturated whilst alkenes are unsaturated
• this makes alkenes susceptible to electrophilic addition
• alkanes need light to react
• by radical substitution

Question 12

Explain the rate of hydrolysis reaction between a halogenoalkane and NaOH (aq) and name the reaction mechanism. Give the equation. (7 marks)

Answer 12

RX + OH‒ → ROH + X‒ (any halogenopropane / NaOH)
mechanism is nucleophilic substitution
C—halogen bond must be broken for the reaction to take place
based on electronegativity, carbon atom in the chlorocompound is the most (partial positive)
suggests that chlorocompound would react fastest
based on bond enthalpies, the C—I bond is the weakest suggests that iodocompound would react fastest
iodocompound does in fact react fastest so effect of bond strength effect is more significant than effect of polarity

Question 13

Draw formation of pi bond in ethene.

Answer 13

Pi orbital above and below plane of molecule (2 horizontal eclipses)

Question 14

Test to see if it is 1-chlorobutane or 1-iodobutane. (3 marks)

Answer 14

Heat with NaOH (aq)
Add acidifies silver nitrate solution
White ppt ->Cl

Question 15

Why is hydrogenation of alkenes to alkanes commercially important?

Answer 15

To change vegetable oil to solid edible fat, e.g. margarine