8.4 - Reactions Of Hydrocarbons

Question 1

What is Toluene?

Answer 1

Methylbenzene

Question 2

What is Combustion?

Answer 2

Exothermic reaction - fuel chemically reacting with O2

Complete

• Organic + O2 —> H2O

Incomplete

• Organic + O2 —> H2O + CO
• Organic + O2 —> H2O + C

Question 3

What are the three hydrocarbon reactions?

Answer 3

1. Combustion
2. Substitution
3. Addition reactions

Question 4

Explain Combustion in Hydrocrabon

Answer 4

• Applies to all hydrocarbons

-

Question 5

Explain Substitution in Hydrocarbons

Answer 5

• Applies to ALKANES and BENZENE ONLY
• a HYDROGEN is SUBSTITUTED for a HALOGEN
  
  • (normal) substitution - 1 hydrogen is subbed (limited amount of halogen)
  • (complete) substitution - all hydrogens are subbed (excess amount of halogen)
• in excess one hydrogen at a time will be replaced
  - HX product will not react again
• React with halogens (X) in presence of UV light
• UV and Heat used as Catalysts
  - slow

Question 6

Explain Addition reactions for hydrocarbons

Answer 6

• Double (and triple bonds) ONLY
• Good for testing if unknown is unsaturated
• Reactant added to double bond
  - H2O = alcohol
  - Halogen = chloroethane
  - O2 + Water = Ethandiol
• Some require catalysts
• Makes polymers from Monosomers
• Quite fast

Question 7

Explain the Markovnikov’s rule

Answer 7

The hydrogen from a HX (a hydrogen halogen reactant) will attach to the carbon with the most hydrogens already attached.

• Part of Addition Reaction

Question 8

What is a substitution reaction?

Answer 8

A chemical reaction where in the presence of UV light a carbon-hydrogen bond is broke. And replaced by a carbon -halogen bond

Question 9

Benzene reaction substitution

Answer 9

Benzene undergoes substitution reaction in which one of the hydrogen atoms is replaced by another atom/group of atoms

Also use UV and Catalyst such as aluminium chloride it aluminium bromide

Question 10

What is an addition reaction?

Answer 10

The double carbon-carbon bond is broken and a new atom or a group of atoms is added to each carbon atom

Question 11

What happens during an addition reaction?

Answer 11

2 reactant molecules combine to form one product molecule

The carbon-carbon double bond ‘opens up’ to become a single bond

The atoms of the small molecule adding to the alkene are ‘added across the double bond’ so that one atom of group from the molecule forms a bond to the carbon atoms on each end of the double bond

No inorganic product formed

All atoms in the reactants end up in the final product

Question 12

Reaction of alkenes with hydrogen

Answer 12

Alkenes react with hydrogen gas in the presence of a metal catalysts
(Eg nickel )to form alkanes

This reaction is known as a hydrogenation reaction and forms a saturated alkane

This reaction is Too slow for the reaction to proceed at room temperature without a catalyst

Question 13

Simplest alkene

Answer 13

Ethane

C2H4

Question 14

Reaction of alkene with halogens

Answer 14

Ethene with bromine

The halogen adds across the double bond of the molecule so in the there is one bromine atom attached to carbon

This reaction proceeds at room temperature without a chayotes,

Other halogens such as Cl2 and I2 undergo addition reactions with alkenes

Question 15

Reactions with alkenes with hydrogen halides

Answer 15

Alkenes also react with hydrogen halides (HCl2, HBr, HF, HI) by addition reactions

Can produce two isomers

Question 16

Reaction of alkenes with water

Answer 16

Alkenes react with water under specific conditions to form the corresponding alcohol

Question 17

How to make alcohol?

Answer 17

Addition reaction of ethene and water using catalyst to increase the rate of reaction.

Eg catalyst - phosphoric acid
At 300°C

Produces ethanol

Gaseous reactants are passed over a solid bed of the catalyst and gaseous ethanol is formed

The reaction of ethene with the steam is described as hydration reaction

Question 18

Hydration reactions

Answer 18

Reactions that involve water as a reactant

In addition reaction, water ‘added’ across the double bond

The reaction is used for the commercial manufacture of ethanol because it is a one-step process that uses little energy apart from initial heating

The catalyst is a solid whereas the reactants and products are gases which means it is assay from the manufactures to remove the product from the reaction mixture, leaving catalyst intact.

Question 19

Test for unsaturation

Answer 19

Bromine is used as a standard test to determine whether a substance contains molecules that are saturated (contain carbon-carbon double bonds).

1. An aqueous bromine solution is orange in colour.
2. When Few drips are added to alkane which is SATURATED- there is NOT IMMEDIATE REACTION and the COLOUR does NOT CHANGE.
3. This is because SUBSTITUTION REACTIONS are SLOW and REQUIRE a CATALYST

However when a few drops of bromine are added to UNSATURATED compound such as an alkene an ADDITION REACTION occurs and the orange colour changed/displayed almost immediately as BROMINE is CONSUMED.