Chemistry of the Alkanes

Question 1

Alkanes are relatively reactive or unreactive?

Answer 1

Unreactive

Question 2

Why are alkanes were called “paraffins”?

Answer 2

Bc alkanes are unreactive.

Question 3

What are the three main types of reactions which alkanes go under?

Answer 3

Combustion, autoxidation and halogenation.

Question 4

What kind of reaction is combustion and what type of compound does it involve?

Answer 4

A high temp rcn with oxygen to give = CO_{<strong>2</strong>} and H₂O

Alkane.

Question 5

What kind of reaction is autoxidation and what type of compound does it involve?

Answer 5

It’s a low temp rcn (<70℃) with oxygen = gives hydroperoxides

Alkanes.

Question 6

What kinf of reaction on alkanese which gives hydroperoxides?

Answer 6

Autoxidation = It’s a low temp rcn (<70℃) with oxygen.

Question 7

Give the general molecular formula of hydroperoxides.

Answer 7

ROOH.

E.g. COOH with single C-O bonds.

Question 8

Give a general reaction of autoxidation.

Answer 8

R-H + O₂ → ROOH

Question 9

How does the reaction of autoxidation occur?

Answer 9

Via a free radical mechanism.

Don’t need to know more details.

Question 10

What kind of processes are autoxidation reactions very important in? Give a few examples.

Answer 10

Decay related processes.

E.g. Perishing of rubber and other organic materials

Decay of food

Ageing of living organisms

Hardening of paint

Question 11

What does the reaction halogenation of alkanes involve?

Answer 11

Halogenation of alkanes = A free radical reaction b/w alkanes and halogens in the presence of UV light.

Question 12

The reaction b/w methane and chlorine under UV. Whst type of reaction is this?

Answer 12

Free radical holgenation.

Question 13

The reaction b/w methane and chlorine under UV is a free radical halogenation. Please illustrate the main reaction.

Answer 13

CH₄ + Cl₂ → (Cl₂ + hv) → CH₃Cl + HCl

Question 14

Why does the rfee radical halogenation reaction b/w methane and chlorine under UV light have relatively little synthetic use?

Answer 14

Bc product mixture contains other hydorgenated derivatives of methane.

e.g. CH₂Cl₂

CHCl₃

CCl₄

And larger alkanes and their choloderivatives.

E.g. ethane and chloroethane

CH₃CH₃

CH₃CH₂Cl

Question 15

Give an example of a reaction of free radical hydrogenation involving an alkane other than methane and their major product.

Answer 15

See lecture 5 notes.

1 H atom removed via Br₂ and hv = replaced by Br to give a 3 (tertiary) major product.

Question 16

Which reaction is faster, ionic or free radical?

Answer 16

Ionic reactions are faster.

Free radical reactions are slower.