OZ - Haloalkanes

Question 1

What are haloalkanes?

Answer 1

Alkanes with at least one halogen atom in place of a hydrogen atom.

Question 2

How do you name a haloalkane?

Answer 2

• Look for the longest carbon chain - this gives you the last part of the compound’s name.
• Add ‘chloro-‘, ‘bromo-‘ or ‘iodo-‘ depending on the type of halogen(s) present. If there’s more than one type, place them in alphabetical order.
• Show the positions of halogen atoms on the carbon chain by including the numbers of the carbon atoms they’re attached to.
• If there’s more than one identical halogen atom, use di- (2), tri- (3) or tetra- (4) before that part of the name.

Question 3

Explain the trend in the boiling points of the haloalkanes as you go down the group

Answer 3

They increase.

Question 4

What do the boiling points of the haloalkanes depend on?

Answer 4

The strength of the intermolecular bonds - the stronger the bonds between the molecules, the higher the boiling point.

Question 5

Explain why the boiling points of the haloalkanes increase down the group

Answer 5

• As you go down group 7 from fluorine to iodine, the atomic radius of the halogen atoms, and the number of electron shells that they have, increases.
• This leads to stronger instantaneous dipole-induced dipole forces between molecules - you have to put in more energy to overcome them.
• So the boiling point of the haloalkanes increases down the group.

Question 6

Is the carbon-halogen bond in haloalkanes polar or non-polar?

Answer 6

Polar.

Question 7

Why are the carbon-halogen bond in haloalkanes polar?

Answer 7

Because fluorine, chlorine and bromine are much more electronegative than carbon.

Question 8

What can electronegative halogens do and what does this mean can happen?

Answer 8

It pulls electron density away from the carbon, so the carbon is electron deficient meaning that it can be attacked by a nucleophile.

Question 9

What does a nucleophile do?

Answer 9

It is an electron-pair donor so it donates an electron pair to somewhere without enough electrons.

Question 10

What are examples of nucleophiles that can react with haloalkanes?

Answer 10

OH-, NH3 and H2O.

Question 11

What is used to show the movement of electrons pairs?

Answer 11

Curly arrows.

Question 12

What do curly arrows show?

Answer 12

The movement of electrons pairs.

Question 13

What can haloalkanes undergo?

Answer 13

Nucleophilic substitution.

Question 14

What is a substitution reaction?

Answer 14

When a functional group in a compound is replaced by another functional group.

Question 15

What happens in a nucleophilic substitution reaction?

Answer 15

A nucleophile attacks a slightly positively charged carbon and replaces the slightly negative atom or group.

Question 16

In what reaction/process do haloalkanes react with hydroxide ions?

Answer 16

By nucleophilic substitution.

Question 17

How do you do a nucleophilic substitution reaction with hydroxide ions? (What are the conditions/processes?)

Answer 17

You have to use warm aqueous sodium hydroxide and do the reaction under reflux, otherwise it won’t work.

Question 18

What is the general equation for the nucleophilic substitution between a haloalkane and hydroxide ions?

Answer 18

R-X + NaOH –> ROH +NaX

Where R represents an alkyl group (an alkane with a hydrogen removed) and the X stands for one of the halogens.

Question 19

What is heterolytic fission?

Answer 19

When both the electrons are taken by one of the atoms.

Question 20

What can water act as?

Answer 20

A nucleophile.

Question 21

What does warming a haloalkane with water result in?

Answer 21

A nucleophilic substitution reaction.

Question 22

Haloalkanes react with ammonia to form what?

Answer 22

Amines.

Question 23

What are amines?

Answer 23

Organic compounds based on ammonia (NH3) but one or more of the hydrogen atoms are replace by alkyl groups.

Question 24

What happens if you warm a haloalkane with excess ethanolic ammonia?

Answer 24

The ammonia swaps place with the halogen - a nucleophilic substitution reaction.

Question 25

What is ethanolic ammonia?

Answer 25

Ammonia dissolved in ethanol.

Question 26

How can you react a haloalkane with ammonia?

Answer 26

Warm a haloalkane with excess ethanolic ammonia.

Question 27

How do amines smell?

Answer 27

Fishy.

Question 28

How do you know if an amine has formed in a reaction?

Answer 28

It may smell fishy.

Question 29

What are the most reactive haloalkanes?

Answer 29

Iodoalkanes.

Question 30

What is the most polar and least polar bond found in haloalkanes?

Answer 30

C-F is the most polar and C-I isn’t polar at all.

Question 31

What experiment can be used to show the reactivity series of haloalkanes?

Answer 31

React the haloalkanes with water in the presence of silver nitrate.

Question 32

Explain how to react haloalkanes with water in the presence of silver nitrate to identify the haloalkanes reactivity series

Answer 32

• You need to see which reacts the fastest.
• Put a chloroalkane, a bromoalkane and an iodoalkane in three different test tubes.
• To each of these add some silver nitrate solution (this contains the water) and some ethanol (as a solvent).
• The silver halide compound is insoluble, so forms a precipitate.
• The precipitate forms fastest with the iodoalkane - so that must be the most reactive. Bromoalkanes react slower than iodoalkanes, and chloroalkanes the slowest of all.

Question 33

What decides the reactivity of a haloalkane?

Answer 33

The carbon-halogen bond.

Question 34

Explain why it’s the carbon-halogen bond that decides the reactivity of a haloalkane

Answer 34

Despite being the most polar, the C-F bond is the strongest - it has the highest bond enthalpy. For a reaction to occur, the carbon-halogen bond needs to break. The stronger that bond is, the slower the reaction will be.

There is faster hydrolysis down the group as bond enthalpy decreases (the bonds are getting weaker).