Organic Mechanisms- Halogenoalkanes

Question 1

State the definition of a nucleophile?

Answer 1

A molecule or substance that has a tendency to donate electrons and forms bonds with positively charged carbon atoms.

Question 2

What are the three common nucleophiles in nucleophilic substitution reactions?

Answer 2

Hydroxide ions (OH-)
Ammonia (NH3)
Cyanide ion (CN-)

Question 3

What is an elimination reaction?

Answer 3

A hydrogen halide is removed to form a double bond so that an alkene is formed

Question 4

What type of reaction occur when halogenoalkane forms a alcohol?

Answer 4

Nucleophilic substitution

Question 5

What type of reaction occur when a halogenoalkane forms a alkene?

Answer 5

Elimination reaction

Question 6

Conditions favouring nucleophilic substitution with KOH?

Answer 6

Warm mixture
Ethanolic

Question 7

What are the three example of nucleophilic substitution reactions with halogenoalkane ( their condition) and what type of product us formed?

Answer 7

(Warmed, add ethanol solution of Aqueous sodium or potassium hydroxide) PRODUCES ALCOHOLS
( Warmed , add alcoholic solution of potassium cyanide) PRODUCES NITRILES
( Excess concentrated ammonia in ethanol under pressure) PRODUCES AMINES

Question 8

Why are they called nucleophilic substitution reactions?

Answer 8

Nucleophile replaces the halogen in the halogenoalkane

Question 9

State the definition of an elimination reaction?

Answer 9

Removing a H+ ion and a halogen halide ion is removed eg Br-
This means an alkene is formed (double bond)

Question 10

Test for alkenes ?

Answer 10

Decolourises bromine water and shows it has a C=C bond

Question 11

Substitution conditions

Answer 11

Hydroxide ions + room temp + dissolved in water (aqueous)

Question 12

Elimination conditions ?

Answer 12

Hydroxide ions + high temp + dissolved in ethanol ( ethanolic)

Question 13

Primary halogenoalkane favour?
( halogen is at the end of hydrocarbon chain)

Answer 13

Substitution reactions

Question 14

Secondary halogenoalkanes favour ?
( halogen is body of the chain )

Answer 14

Both substitution and elimination

Question 15

Tertiary halogenoalkane favour ?
( halogen is at a branch in the chain)

Answer 15

Elimination

Question 16

How does the rate of reaction of the nucleophilic substitution reaction with halogenoalkane change as you go down group 7?

Answer 16

Faster substitution as bond enthalpy decreases ( bonds become weaker )
Rate of reaction increases

Question 17

Which undergoes substitution quicker C-F or C- Br?

Answer 17

C-Br is faster as bonds are weaker so undergo substitution quicker

Question 18

Why is the Carbon - hydrogen bond in halogenoalkanes polar?

Answer 18

Halogens are much more electronegative ( delta negative) than carbon ( delta positive)

Question 19

What makes carbon prone to attack from nucleophiles ?

Answer 19

Delta positive
Electron deficiency so attracts nucleophile ( electron pair donor)