14. Hydrocarbons

Question 1

reactions (reagents and conditions) by which alkanes can be produced:

Answer 1

(a) addition of hydrogen to an alkene in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat.

(b) cracking of a longer chain alkane, heat with Al2O3.

Question 2

(a) the complete and incomplete combustion of alkanes

(b) the free-radical substitution of alkanes by Cl2 or Br2 in the presence of ultraviolet light, as exemplified
by the reactions of ethane

Question 3

mechanism of free-radical substitution

Answer 3

initiation-
propagation-
termination-

Question 4

cracking can be used

Answer 4

cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions

Question 5

unreactivity of alkanes

Answer 5

unreactivity of alkanes, including towards polar reagents in terms of the strength of the C–H bonds and their relative lack of polarity

Question 6

environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons from combustion of alkanes in the internal combustion engine and of their
catalytic removal

Question 7

reactions (including reagents and conditions) by which alkenes can be produced:

Answer 7

(a) elimination of HX from a halogenoalkane by ethanolic NaOH and heat.

(b) dehydration of an alcohol, by using a heated catalyst (e.g. Al 2O3) or a concentrated acid (e.g. concentrated H2SO4).

(c) cracking of a longer chain alkane.

Question 8

reactions of alkenes: electrophilic addition

Answer 8

(i) hydrogen in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat- alkane.

(ii) steam, H2O(g) and H3PO4 catalyst- alcohol.

(iii) a hydrogen halide, HX(g), at room temperature- halogenoalkane.

(iv) a halogen, X2- halogenoalkane

Question 9

oxidation by cold dilute acidified KMnO4

Answer 9

form the diol

Question 10

oxidation by hot concentrated acidified KMnO4

Answer 10

rupture of the carbon–carbon
double bond.

on the carbon-
1. ketone- 2 alkyl groups
2. aldehyde- 1 alkyl group, 1 H
3. CO2&H2O- 2H

Question 10

addition polymerisation

Answer 10

reactions of ethene and propene.

monomer-
repeat unit-
polymer-

Question 11

presence of a C=C bond

Answer 11

aqueous bromine brown to colourless

Question 12

mechanism of electrophilic addition

Answer 12

using bromine/ethene and
hydrogen bromide/propene as examples.

arrow from double bond

Question 12

explain the inductive effects of alkyl groups on the stability of primary, secondary and tertiary cations formed during electrophilic addition (markovnikov addition)