Ch.16 - Organic Chemical Reactions

Question 1

What is the evidence for free radical substitution(monochlorination).

Answer 1

Ethane/butane is present in the resulting mixture. The addition of known sources of free radicles like tetraethyl lead speeds up the reaction. The reaction will occur when only U.V. light is used.

Question 1

Outline the mechanism for free radical substitution.

Answer 1

Free radical substitution has 3 steps: initiation, propagation and termination. It is a chain reaction. Initiation occurs by homolytic fission.

Question 2

Describe the acidity of ethanol.

Answer 2

Ethanol is a weak acid.

Question 3

How does carboxylic acid and alcohol react?

Answer 3

Carboxylic acid and methanol react to produce a water and an ester.

Question 4

What is an addition reaction? Geometry, bonds, benzene.

Answer 4

An addition reaction is one in which 2 substances react to form a single substance. It changes structure from planar to tetrahedral(unsaturated to saturated). Always occurs on double or triple bonds. Cannot happen to benzene as molecule is too stable.

Question 5

What is the use of addition reactions?

Answer 5

Hydrogenation(the addition of H2) of vegetable oils forms margarine. Polymerisation reactions form long chains of carbons called plastics.

Question 6

What are the 4 steps to addition reactions? Name a catalyst.

Answer 6

The 4 steps for addition reaction include polarisation of double bond, heterolytic fission, carbonium ion formation and ionic addition. Nickel can be used as a catalyst.

Question 7

What are polymerisation reations?

Answer 7

Polymerisation reactions are addition reactions in which long chain molecules are made from repeating monomers.

Question 8

What is a substitution reactions?

Answer 8

A substitution reaction is a reaction where an atom(s) is replaced with another atom(s). There is no change in structure.

Question 9

Give 3 examples of substitution reactions.

Answer 9

Halogenation of alkanes, free-radical substitution and esterification reactions are substitution reactions.

Question 10

How are alkanes halogenated? Give an example.

Answer 10

The halogenation of alkanes involves substituting H atoms for halogens. CCl4 is used as a flame retardant(damages the ozone layer).

Question 11

What is esterification?

Answer 11

An ester is formed when a carboxylic acid is reacted with an alcohol, with H2SO4 as a catalyst. Ethanoic acid reacts with ethanol to form ethyl ethanoate and water. This is a reversible reaction, H+ ions can be used as a catalyst(forward reaction needs an acid while reverse needs a base).

Question 12

What is hydrolysis?

Answer 12

Hydrolysis is the chemical decomposition of a substance by water, the water itself also being decomposed.

Question 13

What is base hydrolysis of an ester?

Answer 13

Esters react with KOH or NaOH(bases) to form alcohol and the salt of the carboxylic acid. It is used in soap manufacturing, the salt is soap. Also known as saponification.

Question 14

What is soap? What is used in its manufacture, what are the results?

Answer 14

Soap are the Na or K salts of long-chain carboxylic acids. Vegetable fat(unsaturated) is used. The alcohol produced is propan-1,2,3-triol(glycerol). The salts are K/Na stearate or soap.

Question 15

How does soap work?

Answer 15

Soap contains long, non-polar carbon chains(hydrophobic) which dissolve oils. It also contains a polar COO-Na+ which dissolves salts and water.

Question 16

What is an elimination reaction?
Geometry. Example.

Answer 16

An elimination reaction is when an unsaturated compound is formed from a saturated compound by the removal of a molecule such as water(dehydration). Tetrahedral to planar. Ethanol to ethene and water.

Question 17

State an example and changes in geometry of elimination reactions.

Answer 17

Geometry changes from tetrahedral to planar as a double bond is formed. The dehydration of ethanol to produce ethene and water is an elimination reaction.

Question 18

Describe the combustion of hydrocarbons.

Answer 18

Hydrocarbons burn in O2 to give CO2 and H2O only.

Question 19

Define oxidation and reduction in hydrocarbons.

Answer 19

In hydrocarbons, oxidation is the addition of O and the removal of H. While reduction is the addition of H and the removal of O.

Question 20

What are oxidising agents for hydrocarbons and what are their indicators?

Answer 20

Acidified sodium dichromate Cr(VI) is orange and gets reduced to Cr(III) which is green. Acidified potassium permanganate Mn(VII) is purple and gets reduced to Mn(II) which is colourless. Reduction is carried out using H2 and Ni catalyst.

Question 21

How is ethanol(primary alcohol) oxidised and how is its product oxidised?

Answer 21

Ethanol is oxidised to produce ethanal and water. Ethanal is oxidised to produce ethanoic acid and water. Hence primary alcohols to aldehydes to carboxylic acids when oxidising.

Question 22

How are secondary alcohols oxidised?

Answer 22

Secondary alcohols get oxidised into ketones. No more oxidation can occur. C with O is not at the end of the molecule.

Question 23

What organic redox reactions take place in everyday life?

Answer 23

Wine left in open air will taste of vinegar. Ethanol oxidises to ethanoic acid. When ethanol is consumed, the liver breaks it down as its a toxin. Ethanol is oxidised to ethanal which is even more toxic.

Question 24

How does ethanal react with acidified potassium permanganate?

Answer 24

Solution goes from purple to colourless as ethanal is oxidised to ethanoic acid. Wouldn’t work with a ketone. Hence, aldehydes can be oxidised to carboxylic acids but ketones cannot.

Question 25

How does ethanal react with Fehling’s reagent?

Answer 25

Solution goes form blue to red as ethanal is being oxidised to ethanoic acid. Wouldn’t work with a ketone. Aldehydes can be oxidised but ketones cannot.

Question 26

How does ethanal react with ammoniacal silver nitrate(silver mirror test)?

Answer 26

Silver nitrate, NaOH and NH3 make Tollens’ reagent. Aldehydes reduce Ag(I) ions on heating to metallic silver. Ethanal is being oxidised to ethanoic acid. Wouldn’t work with a ketone. Aldehydes can be oxidised but ketones cannot.

Question 27

Explain the acidic nature of carboxylic acids. Why is the H on the -COOH group acidic? Why can it dissociate?

Answer 27

Because the O in C=O is strongly electronegative, the C is left with a positive charge. The C attracts electrons away from the -OH, allowing H to dissociate, known as the inductive effect. When the hydrogen of the -COOH dissociates to give a H+ ion, COO- is left. The negative charge is delocalised between the 2 O atoms. This gives the structure more stability, allowing the H+ ion to dissociate.

The inductive effect and the stability of the carboxylate ion

Question 28

What type of chemical produces effervescence?

Answer 28

An acid