Haloalkanes

Question 1

Reaction Map

Answer 1

Question 2

General formula

Answer 2

CnH2n+1 X

Question 3

Classification

Answer 3

• In a primary haloalkane the halogen is attached to a C, which is only attached to one other C.
• In a secondary haloalkane the halogen is attached to a C, which is only attached to two other C’s.
• In a tertiary haloalkane the halogen is attached to a C, which is attached to three other C’s.

Question 4

FREE RADICAL definition

Answer 4

A free radical is a species which has a single unpaired electron

Question 5

Why is UV light needed to start the reaction?

Answer 5

UV light provides the energy needed to break THE halogen covalent bond at the start of the reaction

Question 6

Free radical substitution

Answer 6

Cl radical = catalyst

Question 7

How can the amount of further substitution be reduced?

Answer 7

An excess of alkane must be used (which uses up all the halide radicals)

Question 8

nucleophile definition

Answer 8

e- pair donor

Question 9

base definition

Answer 9

accepts H+

Question 10

Why are haloalkanes more reactive than alkanes

Answer 10

• haloalkanes contain polar bonds
• halogen has greater electronegativity than C atom

Question 11

Nucleophilic Substitution by Hydroxide Ions (alcohol)

Answer 11

Reagent: NaOH (aq)

Conditions: Aqueous NaOH, Reflux (warm)

Equation: C2H5 Br + NaOH → C2H5OH + NaBr

Question 12

Nucleophilic Substitution by Cyanide Ions (nitrile)

Answer 12

Question 13

Nucleophilic Substitution with Ammonia (amine)

Answer 13

Conditions: Excess concentrated ammonia, ethanol solvent, heat and pressure.

C- Br has a difference in electronegativity
Cδ+ — Brδ-
lone pair on NH3 attracted to Cδ+

Question 14

Elimination Reaction

Answer 14

Reagent: Concentrated NaOH / KOH(aq)

Conditions: Hot, ethanol solvent
(ethanolic conditions)

Equation: CH3CHBrCH3 + NaOH → CH2CHCH3 + NaBr + H2O

Question 15

Hydrolysis definition

Answer 15

Splitting molecules using water

Question 16

Method to see if hydrolysis has occurred

Answer 16

1. Place a measured sample of each halogenoalkane, into separate test tubes and place in to a water bath at 50℃
2. Into another test tube place a solution of ethanol, water and aqueous silver nitrate and place in the same water bath.
3. Once at equal temperature add equal volumes of the ethanol mixture and haloalkane solutions together.
4. Time how long it takes for each precipitate to form.

The rate of this reaction can be determined by calculating 1/time taken for the precipitate to occur

Question 17

Reason for the differing rates of hydrolysis

Answer 17

The stronger the bond the harder it is to break and the slower the rate of reaction will be as more energy is required to break it.

The weaker the bond the faster the rate will be.

C – I - weakest bond so fastest reaction, less energy require to break the bond

C – Cl - strongest bond so slowest reaction, more energy required to break the bond

Question 18

The Ozone layer

Answer 18

Ozone is formed naturally in the upper atmosphere (stratosphere) by free radical reactions.

O2 is split into two by ultraviolet radiation into two oxygen atoms and then one of the oxygen atoms combines with an oxygen molecule to form ozone.
O2 → 2O▪
O2 + O ⇌ O3

Radicals can disrupt this balance, as they can catalyse the reaction to form oxygen

Question 19

Uses of Haloalkanes

Answer 19

CFC’s were used extensively as they have useful properties
* non-toxic
* inert gases
* volatile

Uses
Refrigeration
Air conditioning
Dry cleaning solvents

Question 20

How CFCs damage the Ozone layer

Answer 20

CFC’s are unreactive and so difficult to breakdown.
However once they reach the stratosphere they are broken down by UV

Alkanes, HFC’s hydrofluorocarbons and HCFC’s hydrochlorofluorocarbons are used frequently.

”ozone friendly” alternatives break down rapidly in the lower atmosphere, as they are more reactive in comparison to CFC’s, hence biodegradable.