Halogenoalkanes

Question 1

What are halogenoalkanes?

Answer 1

Have an alkane skeleton with one or more halogen atoms in place of hydrogen atoms

Question 2

Describe the physical properties of halogenoalkanes (solubility)

Answer 2

The polar C—X bonds are not polar enough to make them soluble in water.

The main intermolecular forces of attraction are dipole-dipole attractions and van der Waal forces.

They mix with hydrocarbons so they can be used as dry-cleaning fluids and to remove oily stains (oil is a mixture of hydrocarbons).

Question 3

Describe the physical properties of halogenoalkanes (boiling point)

Answer 3

Depends on number of carbon atoms and halogen atoms.

Boiling point increases with increased chain length and increases going down the halogen group.

Both these effects are caused by increased vdW forces because the larger the molecules, the greater the number of electrons (and therefore the larger the vdWs forces)

Question 4

Why do halogenoalkanes have higher boiling points than alkanes with similar chain lengths?

Answer 4

Because they have higher relative molecular masses and they’re more polar

Question 5

What two factors determine how readily the C—X bond break during the reaction of halogenoalkanes?

Answer 5

The C—X bond polarity

The C—X bond enthalpy

Question 6

Describe the bond polarity of halogenoalkanes

Answer 6

Halogens are mire electronegative than carbon so the bond polarity will be Cs+—Xs-

This means that the carbon bonded to the halogen has a partial positive charge (it is electron deficient)

This means that it can be attacked by reagents that are electron rich or have electron-rick areas (nucleophiles)

Question 7

What does the bond polarity of the halogenoalkane C—X bond predict in terms reactivity?

Answer 7

The C—F bond would be the most reactive as it is the most polar (so the Cs+ has the most positive charge and is therefore most easily attacked by a nucleophile).

The C—I bond would be least reactive because it is least the least polar.

Question 8

Describe bond enthalpies of halogenoalkanes

Answer 8

The bond gets weaker going down the group.

Fluorine is the smallest atom of the halogens and the shared electrons in the C—F bond are strongly attracted to the fluorine nucleus. This makes a strong bond. Going down the group, the shared electrons in the C—X bond get further and further away from the halogen nucleus, so the bond becomes weaker.

Question 9

What does the bond enthalpies of the halogenoalkane C—X bond predict in terms reactivity?

Answer 9

That iodo-compounds, with the weakest bond, are the most reactive, and fluoro-compounds, with the strongest bonds, are the least reactive.

Question 10

What have experiments confirmed about the reactivity of C—X bonds?

Answer 10

Reactivity increases going down the halogen group.

This means that bond enthalpy is a more important factor than bond polarity.

Question 11

Define a nucleophile

Answer 11

Species that has a lone pair of electrons with which it can form a bond by donating its electrons to an electron deficient carbon atom.

Question 12

Name 3 different nucleophiles

Answer 12

• :OH
• :CN

:NH3

Question 13

Name 3 reagents that could be used for nucleophilic sibstitution

Answer 13

Sodium or potassium hydroxide

Potassium cyanide

Ammonia

Question 14

Draw the mechanism for nucleophilic substitution of bromoethane with sodium hydroxide

Answer 14

Book (P209)

Question 15

What does nucleophilic substitution with hydroxide ions usually form?

Answer 15

An alcohol + a halide ion

Question 16

Draw the mechanism for nucleophilic substitution of bromoethane with potassium cyanide

Answer 16

Book (P209)

Question 17

What does nucleophilic substitution with cyanide ions usually form?

Answer 17

Nitrile + halide ion

Question 18

What does nucleophilic substitution with ammonia usually form?

Answer 18

An amine + ammonium halide

Question 19

Draw the mechanism for nucleophilic substitution of bromoethane with ammonia

Answer 19

Book (P210)

Question 20

Why is ammonia a nucleophile?

Answer 20

Because it has a lone pair of electrons hat it can donate (although it has no negative charge) and the nitrogen atom has a s- charge

Question 21

Draw a diagram to show the uses of nucleophilic reactions

Answer 21

Book (P210)

Question 22

Outline what happens during the elimination reactions in halogenoalkanes

Answer 22

A hydrogen halide is eliminated from the parent molecule, leaving a double bond in its place so that an alkene is formed

Question 23

Draw the mechanism for elimination reaction of bromoethane with potassium hydroxide

Answer 23

Book (P211)

Question 24

What does elimination reaction of halogenoalkanes (with potassium hudroxide) usually produce?

Answer 24

Alkene, potassium halide, water

Question 25

What conditions does elimination reaction of halogenoalkanes prefer?

Answer 25

The sodium or potassium hydroxide is dissolved in ethanol and mixed with the halogenoalkane. The mixture is heated.

Question 26

Describe what happens during elimination reaction of halogenoalkanes.

Answer 26

The OH- ion used its lone pair to form a bond with one of the hydrogen atoms on the carbon next to the C—X bond. These hydrogen atoms are very slightly s+.

The electron pair from the C—H bond now becomes part of a carbon-carbon double bond.

The halogen takes the pair of electrons in the C—X bond and leaves as a halide ion (the leaving group).

Question 27

How are halogenoalkanes classified?

Answer 27

According to whether the halogen atom is at the end of the hydrocarbon chain (primary), in the body of the chain (secondary) or at a branch in the chain (tertiary)

Question 28

Describe how halogenoalkanes affect the environment (with a specific example)

Answer 28

CFCs:

Unreactive under normal conditions. But CFC gases eventually end up in the atmosphere where they decompose to give chlorine atoms. Chlorine atoms decompose ozone, in the stratosphere, which has caused a hole in the Earth’s ozone layer.