6D: Halogenoalkanes

Question 1

What is a halogenoalkane?

Answer 1

An alkane with at least one halogen atom in place of a hydrogen atom

Question 2

What are the three types of halogenoalkanes?

Answer 2

Primary Secondary Tertiary

Question 3

What is a primary halogenoalkane?

Answer 3

Carbon with the halogen only bound to one/no alkyl groups

Question 4

What is a secondary halogenoalkane?

Answer 4

Carbon with the halogen is bound to two alkyl groups

Question 5

What is a tertiary halogenoalkane?

Answer 5

Carbon with halogen is bound to three other alkyl groups

Question 6

How are halogenoalkanes turned into alcohols?

Answer 6

Hydrolysed by nucleophilic substitution reaction Uses water

Question 7

What is the general equation for the hydrolysis of halogenoalkanes to alcohols?

Answer 7

R-X + H2O -> R-OH + H+ + X-

Question 8

How can you test the reactivity of halogenoalkanes?

Answer 8

Add water and silver nitrate

Water causes halide ions which reacts to form precipitate

Add both to a halogenoalkane and time how long to form a precipitate

Faster the precipitate forms, the faster the rate of hydrolysis is for that halogenoalkane

Question 9

What is required to hydrolyse a halogenoalkane?

Answer 9

Break the carbon-hydrogen bond

Question 10

What does bond enthalpy have to do with rate of hydrolysis?

Answer 10

The lower the bond enthalpy - the lower the bond strength Therefore it reacts faster

Question 11

What halogens have the weaker C-X bond?

Answer 11

Iodine has the weakest bond As size of halogen is largest the C-X bond is longer and therefore weaker

Question 12

What are the reactivities of primary to tertiary halogenoalkanes?

Answer 12

Tertiary most reactive

Question 13

Why is the carbon-halogen bond polar?

Answer 13

Halogens are generally more electronegative than carbon

Question 14

What can the carbon bonded to a halogen be attacked by and why?

Answer 14

Nucleophile Carbon is electron-deficient (slight positive charge)

Question 15

Name some nucleophiles that attack halogenoalkanes?

Answer 15

OH- NH3 CN- Water is a weak nucleophile

Question 16

What are the two forms of nucleophilic substitution and when do they occur?

Answer 16

SN1 - nucleophilic substitution 1 ( 1 species in rate determining step) Occurs with tertiary halogenoalkanes SN2- nucleophilic substitution 2 ( 2 species in rate determining step) Occurs with primary halogenoalkanes

Question 17

What is the reaction to produce alcohols from halogenoalkanes?

Answer 17

Warm aqueous potassium hydroxide and do reaction under reflux Water not used as it is a worse nucleophile

Question 18

What is the reaction between cyanide ions and halogenoalkanes?

Answer 18

Reflux a halogenoalkane with potassium cyanide in ethanol to form a nitrile

Question 19

What is the reaction between halogenoalkane and cyanide ions?

Answer 19

R-X + CN^- → R≡CN + X^-

Triple bond between carbon in nitrile group and the carbon in R group

Question 20

What is the formula for an amine?

Answer 20

R-NH₂

Question 21

What is the reaction general formula forming amines from halogenoalkanes?

Answer 21

R-X + NH₃ -> R-NH₂ + NH₄⁺ + X^-

Question 22

What are the coniditions for the formation of amines from primary halogenoalkanes?

Answer 22

Warm a halogenoalkane with ammonia dissolved in ethanol

Question 23

What do amines smell like?

Answer 23

Fishy smells

Question 24

What happens when halogenoalkanes react with ethanolic alkali (e.g. KOH)?

Answer 24

Alkene, water and bromide forms

C₂H₅Br + KOH -> C₂H₄ + H₂O + KBr

Question 25

What are the conditions and reactants for an elimination reaction with a halogenoalkane?

Answer 25

Warm alkali dissolved in ethanol (e.g. KOH)

Heated under reflux

Question 26

What do the hydroxide ions do in the elimination?

Answer 26

Hydroxide ions act a as a base to remove H⁺ form the halogenoalkane to form an alkene

Question 27

Why is it useful to substitute a halogen with cyanide ion?

Answer 27

Increases the carbon chain by one