Halogenoalkanes

Question 1

What is a halogenoalkane

Answer 1

An organic compound that contains a halogen

Question 2

What is the general formula of mono-halogenoalkanes

Answer 2

CnH2n+1X

Question 3

How do you name halogenoalkanes

Answer 3

Same as alkanes but with halogeno-prefix

Question 4

What is the name of CH3CH2F

Answer 4

Fluoroethane

Question 5

What is the name of CH3CHBrCHBrCHICCl3

Answer 5

3,4-Dibromo-1,1,1-trichloro-2-iodopentane.

alphabetic order

Question 6

How are halogenoalkanes classified as primary, secondary or tertiary

Answer 6

According to the number of carbon atoms binding to the carbon atom that the halogen is attached to (like alcohols)

Question 7

What are the physical properties of halogenoalkanes

Answer 7

They are subject only to van der Waals forces, but they have permanent dipoles so the attraction is stronger than in alkanes.

Question 8

Why do halogenoalkanes have higher boiling points that alkanes

Answer 8

Because halogenoalkanes have permanent dipoles so the attraction is stronger

Question 9

Which halogens giver higher boiling points

Answer 9

Larger halogens

I>Br>Cl>F

Question 10

Why is F the worst group to leave

Answer 10

It is around the same size as H, C-F bonds are strong so it is the worst group to leave

Question 11

Why is the size of the halogen a determinate of the reactivity

Answer 11

The larger the halogen the weaker the X-C bonds so the better a leaving group so the more reactive

Question 12

How can halogenoalkanes be formed

Answer 12

1. ) From alcohols.
2. ) By electrophilic addition.
3. ) By free radical substitution.

Question 13

What is free radical substitution

Answer 13

A chain reaction that involves initiation, propagation and termination

Question 14

What reaction happens during initiation

Answer 14

Cl2 -> 2Cl·

Question 15

What is homolytic fission and when does it occur

Answer 15

Homolytic fission is where one electron goes to each of the molecules previously bound. Happens in initiation (one electron goes to each Cl)

Question 16

What reactions happen during propagation

Answer 16

Cl· + H-CH3 -> ·CH3 + HCl

Then ·CH3 + Cl2 -> CH3Cl + Cl·

Question 17

What reactions occur during termination

Answer 17

2Cl· -> Cl2
and ·CH3 + Cl· -> CH3Cl
and 2·CH3 -> CH3CH3

Question 18

During which stages of initiation, propogation and termination can the product of the reaction be formed

Answer 18

Propagation and termination

Question 19

What is the basis of ozone depletion by CFCs

Answer 19

The C-Cl bond is easily broken by UV light:

CH3-Cl (+uv) -> ·CH3 + Cl·

Question 20

What is the name of CHCl3 and give a use of it

Answer 20

Chloroform (trichloromethane). Makes a good anaesthetic

Question 21

What does electrophilic addition of HX to alkenes involve

Answer 21

Involves the protonation of an alkene then the attack of a halogen

Question 22

Draw the mechanism of electrophilic addition of HX to alkenes

Answer 22

Goes to the more substituted form

Question 23

How do alkyl groups help to stabilise carbocations

Answer 23

They spread the positive charge

Question 24

What is the hierarchy of stability of carbocations

Answer 24

Primary > Secondary > Tertiary

Question 25

Which carbocation is the most stable

Answer 25

Tertiary

Question 26

What is a carbocation

Answer 26

A carbon with a positive charge

Question 27

Draw a primary carbocation

Answer 27

ethyl

Question 28

Draw a secondary carbocation

Answer 28

2-propyl

Question 29

Draw a tertiary carbocation

Answer 29

Diagram

Question 30

How do you form a halogenoalkane from alcohols

Answer 30

Treat the alcohol with PX3 or with HX (HCl, HBr, HI)

Question 31

What is Sn2

Answer 31

Bimolecular nucleophilic substitution

Question 32

Describe the mechanisms of the reaction of a primary alcohol by Sn2

Answer 32

The alcohol is protonated (fast). protonated alcohol is attacked by Cl- (nucleophile) forms halogenoalkane and water (slower). The Sn2 part of the reaction is when the protonated alcohol is attacked by a nucleophile

Question 33

What are the molecules involved in the rate determining steps of Sn2

Answer 33

Protonated alcohol and halogen nucleophile

Question 34

What are the two simultaneous processes of Sn2

Answer 34

Attack with alcohol and loss of water

Question 35

What does the rate of the Sn2 reaction depend on

Answer 35

The concentrations of both the alcohol and the acid

Question 36

How do primary alcohols react

Answer 36

By Sn2

Question 37

How do tertiary alcohols react

Answer 37

By Sn1

Question 38

Why do tertiary alcohols react by Sn1

Answer 38

Because the central carbon is not accessible to nucleophilic attack

Question 39

Describe the mechanism of a tertiary alcohol reacting by Sn1

Answer 39

A proton attacks the OH group on the tertiary alcohol (fast). The C-O bond breaks, a water molecule and tertiary carbocation form (very slow). The carbocation is attacked by a nucleophile (halogen) due to the shape of the carbocation making it accessible (fast). A halogenoalkane is formed.

Question 40

What does the rate of reaction depend on

Answer 40

The concentration of the alcohol as only alcohol is present in the slow step.

Question 41

What does the nucleophile attack in Sn2

Answer 41

Accessible δ+C

Question 42

What does the nucleophile attack in Sn1

Answer 42

A stabilised carbocation (C+)