Halogenoalkanes And Nucleophilic Substitution

Question 1

What is a halogenoalkane

Answer 1

Compound in which one or more of the hydrogen atoms is replaced by a halogen

Question 2

Why are halogenoalkanes reactive

Answer 2

As the carbon in the carbon-halogen bond is partially positive meaning it is easily attacked by nucleophiles

Question 3

What type of bond breaking is it with the carbon halogen bond

Answer 3

Heterolytic fission with the halogen taking both bonding electrons to form a negative ion

Question 4

What is a nucleophile

Answer 4

An electron pair donor so has a negative charge

Question 5

Four examples of nucleophiles

Answer 5

OH-, H2O, NH3, CN-

Question 6

What is hydrolysis

Answer 6

The breaking of a bond in a reaction involving water

Question 7

How do the carbon-halogen bond reactivity change down the group and why

Answer 7

Get more reactive as you go down group so react faster as bond becomes weaker(bond enthalpy decreases) so requires less energy to break due to increase in number of inner electron shells(increasing shielding) and increase in atomic radius which decreases force of electrostatic attraction between bond pair and halogen nuclei

Question 8

Why does reactivity increase from primary to secondary to tertiary carbocation

Answer 8

As more alkyl groups are added there is more of a positive inductive effect which stabilises the positive charge on the carbon atom so tertiary carbocation most stable and reactive

Question 9

Why do we test for carbon-halogen bond using OH- instead of H2O

Answer 9

As hydrolysis with water is slow so use OH- as nucleophile in form sodium hydroxide

Question 10

Steps for test for halogenoalkanes

Answer 10

-add drop of ethanol to test tube(so halogenoalkane dissolves) and add halogenoalkane
-add sodium hydroxide and heat under reflux
-acidify with dilute nitric acid
-add silver nitrate solution and begin stopclock, stop when precipitate forms and observe its colour

Question 11

What is heating under reflux

Answer 11

Heating with a reflux condenser placed vertically in the flask to prevent vapour escaping while liquid is being heated so the vapour from the boiling reaction mixture condenses and flows back into the flask

Question 12

What does substitution reaction with cyanide ions produce

Answer 12

A nitrile compound(eg butanenitrile) and halogen ion

Question 13

Conditions for substitution reaction by cyanide ions

Answer 13

Heat halogenoalkane under reflux with potassium cyanide in ethanol

Question 14

Why use reflux condenser in substitution reaction by cyanide ions

Answer 14

Ensures that the volatile substances (halogenoalkane and ethanol) condense and drip back into the reaction mixture and prevents the release of toxic vapours from these

Question 15

Why is substitution reaction by cyanide ions a useful one

Answer 15

Reaction useful way of increasing the length of the carbon chain in a molecule

Question 16

What are the two steps of substitution reaction with ammonia

Answer 16

1st: produces a primary amine(nitrogen of amine group is attached to one alkyl group)
2nd: in presence of excess ammonia it reacts with two ammonia molecules to form an ammonium salt with the halogen ion that is released eg ammonium bromide

Question 17

Conditions for substitution by reaction with ammonia

Answer 17

Warm halogenoalkane with a concentrated solution of ammonia in ethanol in a sealed tube

Question 18

Why can there be mixing of products in substitution reaction with ammonia

Answer 18

As there is not only a lone pair on the ammonia but also a lone pair on the nitrogen in the primary amine which has an increased reactivity due to the inductive effect of the alkyl group it is attached to meaning the primary amine can react with the halogenoalkane and lead to further mixing of products

Question 19

How to limit further reactions in substitution by reaction with ammonia

Answer 19

Use excess ammonia to increase the chance of ammonia(not amine as there is greater proportion of ammonia than primary amine) acting as nucleophile with the halogenoalkanes

Question 20

Conditions for hydroxide ions as nucleophiles

Answer 20

Add aqueous sodium hydroxide in aqueous ethanol solvent and heat under reflux

Question 21

What is produced when halogenoalkanes nucleophilic substitution with hydroxide ions

Answer 21

Alcohol and halide ions

Question 22

Why use a sealed tube in the reaction with ammonia

Answer 22

To prevent the escape of toxic ammonia

Question 23

Why is primary amine a stronger nucleophile than ammonia

Answer 23

Due to positive inductive effect of alkyl group which release electrons so the electron density on the nitrogen is higher compared to ammonia

Question 24

What is a base

Answer 24

A proton acceptor

Question 25

Why is rate of reaction faster with hydroxide ions than with water

Answer 25

As the negatively charged oxygen on the hydroxide ion has a greater electron density than the oxygen atom of a neutral water molecule

Question 26

How to name amine

Answer 26

Alkane suffix + yl and amine eg butyl amine

Question 27

Conditions for substitution with water

Answer 27

Heat under reflux with water and ethanol