Halogenoalkanes

Question 1

Halogenoalkane

Answer 1

Alkane with at least 1 halogen atom in place of a hydrogen atom
Can be more than one halogen

Question 2

What can halogenoalkanes with just 1 halogen atom form?

Answer 2

Primary, secondary or tertiary halogenoalkanes

Question 3

Primary halogenoalkane

Answer 3

On the carbon which has a halogen attached, it has:
2 hydrogen atoms
1 alkyl group

Question 4

Secondary halogenoalkane

Answer 4

On the carbon which has a halogen attached, it has:
1 hydrogen
2 alkyl groups

Question 5

Tertiary halogenoalkane

Answer 5

On the carbon which has a halogenoalkane attached, it has:
3 alkyl groups

Question 6

Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 1

Answer 6

Polar bond between carbon and halogen in the halogenoalkane where carbon is d+ and halogen is d-

Question 7

Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 2

Answer 7

Nucleophile (electron pair donor) is attracted to the C d+
(Could be H2O, OH- etc)

Question 8

Mechanism for hydrolysis of halogenoalkane (nucleophilic substitution) step 3

Answer 8

The halogen is substituted for nucleophile
Because heterolytic fission of carbon-halogen bond occurs to the halogen atom forming halogen ion and nucleophile bonds to C d+

Question 9

Examples of nucleophilic substitution of halogenoalkanes

Answer 9

Water in hydrolysis = alcohol
Aqueos KOH = alcohol
Cyanide ions = nitriles
Ammonia = amines

Question 10

How can halogenoalkanes form alcohol?

Answer 10

In hydrolysis (add water)

Question 11

What is the hydrolysis of halogenoalkanes called?

Answer 11

Nucleophilic substitution

Question 12

General equation for hydrolysis of halogenoalkane (words)

Answer 12

Halogenoalkane + OH- or H2O
Will form:
Halogen ion
(+ metal ion if alkali hydroxide)
Hydrogen ion
And an alcohol with remaining atoms

Question 13

Adding water to halogenoalkane mechanism

Answer 13

H2O splits: OH- ion substitutes halogen atom in halogenoalkane forming an alcohol, a halide ion and H+ ion

Question 14

How to compare reactivities of halogenoalkanes using hydrolysis

Answer 14

Mix halogenoalkane with water = the halide ions substituted form which react with silver nitrate solution added to form silver halide precipitates
Time how long precipitate takes to form because this can show faster rate of hydrolysis

Question 15

What determines the rate of hydrolysis of halogenoalkanes?

Answer 15

The carbon - specific halogen bond enthalpy
Weaker bonds break easier

Question 16

What determines bond enthalpy of carbon-halogen bond?

Answer 16

Larger halogen radii causes longer carbon to halogen bond so a lower bond enthalpy so a weaker bond

Question 17

What halogenoalkane has the fastest rate of hydrolysis ?

Answer 17

Iodoalkanes

Question 18

If we want to speed up hydrolysis of halogenoalkane to alcohol, what can we do instead?

Answer 18

Use a aqueos hydroxide solution eg KOH because there are OH- ions which are stronger nucleophile

Question 19

Hydroxide ion nucleophilic substitution

Answer 19

C d+ attracts lone pair of electrons from OH- ion
Causing heterolytic fission of C-halogen bond
So new bond = OH group bonds to carbon and halogen ion is formed

Question 20

What are all the nucleophilic substitution reactions with halogenoalkanes?

Answer 20

+ Water/ OH- ions = alcohol
+ ammonia ion = amine

Question 21

What are amine compounds called?

Answer 21

Normal naming system + amine as functional group

Question 22

Step 1 of amine + halogenoalkane

Answer 22

Dipole across Cd+ and Xd-
Lone pair of electrons on nitrogen on NH3 is attracted to d+ carbon
Heterolytic fission of C-X bond

Question 23

Step 2 of ammonia + halogenoalkane

Answer 23

Forms NH3 in place of halogen and halogen is removed to be an ion

Question 24

Step 3 of ammonia + halogenoalkane

Answer 24

Nitrogen can only form 3 bonds but is bonded to 3 hydrogen and carbon so is electron deficient (becomes N+)

Question 25

Step 4 of ammonia + halogenoalkane

Answer 25

Another ammonia molecule has nitrogen’ lone pair of electrons attracted to H d+ (dipole across N-H bond)

Question 26

Step 5 of ammonia + halogenoalkane

Answer 26

Heterolytic fission of N-H bond where electrons from H goes to N
And H+ ion forms dative bond to the ammonia molecule - ammonium ion

Question 27

Product of ammonia + halogenoalkane

Answer 27

An ammonia ion
A halogen ion
And an amine where NH2 is replacing halogen atom which has lone pair of electrons on N

Question 28

Conditions for ammonia + halogenoalkane

Answer 28

Heated in ethanol conditions in a sealed tube (ammonia is a gas)
EXCESS AMMONIA

Question 29

What happens if ammonia is limiting?

Answer 29

A mixture of products may form
Because amine has lone pair of electrons so is a strong nucleophile

Question 30

Elimination reactions of halogenoalkane with an ethanolic alkali

Answer 30

Forms an Alkene by eliminating hydrogen + halogen atoms
Forms water from OH- and H+ removed from halogenoalkane
Forms salt from halogen atom+ metal in alkali

Question 31

Elimination reaction products

Answer 31

An Alkene
Water
A salt

Question 32

Conditions for elimination reaction

Answer 32

the alkali is dissolved in ethanol
Reflux

Question 33

Halogenoalkane + aqueous alkali
Vs
Halogenoalkane + ethanolic alkali

Answer 33

Aqueos alkali forms an alcohol in nucleophilic substitution
Ethanolic alkali forms an Alkene in elimination reaction

Question 34

Cyanide ions are?

Answer 34

CN-

Question 35

Halogenoalkane + cyanide ions forms?

Answer 35

A nitrile which is the carbon chain + C≡N
+ halide ions

Question 36

When adding cyanide ions to a halogenoalkane what happens to the carbon chain?

Answer 36

Increases by 1 because C is added from cyanide ions