Topic 17 Organic 2

Question 1

What is structural isomerism?

Answer 1

When compounds have the same molecular formula but different structural formulas.

E.g. both have C₃H₇Cl
but one is CH₃CH₂CH₂Cl and the other is CH₃CHClCH₃

Question 2

What is stereoisomerism?

Answer 2

Where molecules have the same structural formula but the atoms are arranged differently in space.

This consists of both E/Z (Cis/Trans) isomerism and optical isomerism.

Question 3

What is chain isomerism?

Answer 3

Where structural isomers have different arrangements of the carbon skeleton.

E.g. 2-methylbutane is a chain isomer of pentane.

Question 4

What is position isomerism?

Answer 4

Where structural isomers have the same functional group attached to the same carbon chain, but in a different position.

E.g. butan-2-ol is a position isomer of butanol.

Question 5

What is functional group isomerism?

Answer 5

Where structural isomers have different functional groups.

E.g. butanol is a functional group isomer of butane.

Question 6

What is Cis/Trans isomerism? What is E/Z isomerism?

Answer 6

Cis/Trans - These are terms used to describe the structure of isomers. Cis is used if both the functional groups attached to each carbon in a C=C are on the same side. Trans is used if both the functional groups attached to each carbon in a C=C are on different sides.

E/Z - This is an extension of Cis/Trans isomerism that is used for more complex molecules. E is entgegen (opposite) and is the same as Trans. Z is zusammen (same) and is the same as Cis. The more complex molecules usually have more than one functional group attached to a C=C and therefore the functional group with the highest priority (highest atomic number) is used to identify the molecule as E/Z.

Question 7

What is a chiral molecule?

Answer 7

A molecule that has a non-superimposable mirror image. This is due to the structure of the two molecules.

E.g. Human hands are mirror images of each other but are non-superimposable.

Question 8

What does superimposable mean?

Answer 8

The ability to place two mirror images on top of each other to make two identical products without rotating/changing either object.

E.g. Hands are mirror images of each other and match if one hand is rotated and placed on top of the other. However, because a hand needs to be rotated, they are not superimposable.

Question 9

What is an asymmetric carbon atom?

Answer 9

A carbon atom that is attached to 4 different functional groups.

E.g. CH₃-CH(OH)-COOH
The carbon atom in CH(OH) is attached to the CH₃, H, OH and COOH functional groups.

Question 10

What is an asymmetric molecule?

Answer 10

A molecule that has no centre, axis or plane of symmetry. All asymmetric molecules are chiral.

Question 11

What is an optical isomer/enantiomer?

Answer 11

When a chiral molecule and its non-superimposable mirror image rotate the plane of plane-polarised light in two different directions (one clockwise and the other anticlockwise).

Question 12

What is a racemic mixture?

Answer 12

A mixture of equal amounts of the two mirror-image forms of a chiral compound. The mixture does not rotate polarised light as the two optical isomers have equal and opposite effects so they cancel out.

Question 13

What is a nitrile?

Answer 13

An organic compound with the C≡N functional group.

E.g. Propionitrile

Question 14

What is nucleophilic addition?

Answer 14

When an electron-rich species, a nucleophile, attacks the electron-deficient carbon atom of the unsaturated C=O bond, leading to the formation of a C-O single bond.

Question 15

What is a nucleophile?

Answer 15

An electron-pair donor.

Question 16

What is a crystalline derivative?

Answer 16

A chemical which can be purified by recrystallisation and then identified by measuring the melting temperature.

Question 17

What is an aldehyde?

Answer 17

An organic compound with the CHO functional group.

This is different from the COH functional group (alcohol) as it contains a C=O bond and C-H bond instead of one C-OH bond.

E.g. Propanal (ane replaced by al)

Question 18

What is the carbonyl group?

Answer 18

A group of organic functional groups which contain the C=O bond. Carbonyl compounds such as aldehydes and ketones only have carbon or hydrogen bonded to the C=O bond.

Other functional groups in the carbonyl group that contain the C=O bond but have other atoms then carbon and hydrogen attached to the C=O bond are not carbonyl compounds. These consist of carboxylic acids, acyl chlorides, esters and amides.

Question 19

What is a ketone?

Answer 19

An organic compound with the C=O bond in the middle (with 2 carbons attached to the carbon in the C=O bond).

E.g. Prop-2-one

Question 20

How are carbonyl compounds (aldehydes and ketones) formed?

Answer 20

Aldehydes are formed by the oxidation of a primary alcohol. Ketones are formed by the oxidation of a secondary alcohol. For both, an excess of alcohol is distilled with potassium or sodium dichromate (VI) and dilute sulphuric acid.

Question 21

How can carbonyl compounds (aldehydes and ketones) be oxidised?

Answer 21

Aldehydes can be oxidised to form a carboxylic acid. For this, acidified potassium dichromate (VI) is added to the aldehyde and the mixture is distilled under reflux. To identify when the carboxylic acid has been formed, there will be a colour change from orange (due to the initial Cr₂O₇²⁻ ions) to green (due to the Cr³⁺ ions).

Ketones can be oxidised further through the use of very strong oxidising agents.

Question 22

What does heating under reflux mean?

Answer 22

This means that no reactant (aldehyde) is removed during the heating process. This is done by the use of a reflux condenser which condenses any gases back into the mixture.

In the case of oxidising aldehydes, due to the difference in boiling points between the aldehyde and carboxylic acid, a clear colour change can be observed when all the aldehyde has been oxidised as the carboxylic acid does not evaporate.

Question 23

How can carbonyl compounds (aldehydes and ketones) be reduced?

Answer 23

Metal hydrides can reduce carbonyl compounds to alcohols. Lithium tetrahydridoaluminate (LiAlH₄) is a powerful reducing agent that can reduce aldehydes to primary alcohols and ketones to secondary alcohols.

Question 24

How can aldehydes and ketones be distinguished?

Answer 24

Because aldehydes can be oxidised with mild oxidising agents but ketones cannot, three mild oxidising agents can be used to distinguish between an aldehyde and a ketone. These are Fehling’s Solution, Benedict’s Solution, and Tollens’ Reagent.

Question 25

What is Fehling’s Solution?

Answer 25

This is a mixture of copper (II) sulphate and 2,3-dihydroxybutanedioate ions in a strong alkali.

By adding this to an aldehyde, an orange-brown precipitate is formed from the copper (I) oxide (Cu₂O).

Question 26

What is Benedict’s Solution?

Answer 26

This is very similar to Fehling’s Solution but more stable. This is because it is less strongly alkaline and does not react, therefore it is reliably with all aldehydes.

By adding this to an aldehyde, an orange-brown precipitate is formed from the copper (I) oxide (Cu₂O).

Question 27

What is Tollens’ Reagent?

Answer 27

This consists of an alkaline solution of diamminesilver (I) ions (Ag(NH₃)₂). This then forms a complex ion with ammonia and produces silver (I) ions.

By adding this to an aldehyde, the silver ions become metallic silver. This silver coats clean glass giving the appearance of a mirror. This is called the silver mirror test.

Question 28

What is the acyl group?

Answer 28

An acyl group consists of organic compounds with the structure of a carboxylic acid without the OH functional group.

E.g. Ethanoyl (CH₃CO⁺)

Question 29

What is an acyl chloride?

Answer 29

An organic compound that is part of the acyl group and has at least 1 Cl atom attached.

E.g. Ethanoyl Chloride (CH₃COCl)

Question 30

What is acylation?

Answer 30

A reaction which substitutes an acyl group for a hydrogen ion. The H atom may be part of an OH group, NH₂ group or a benzene ring.

E.g. Ethanoyl Chloride + Propan-1-ol to form an ester and HCl

CH₃COCl + CH₃CH₂CH₂OH –> CH₃COOCH₂CH₂CH₃ + HCl

Question 31

What is a condensation reaction?

Answer 31

A reaction in which molecules join together by splitting off a small molecule such as water.

Question 32

What is a condensation polymer?

Answer 32

A polymer formed by a series of condensation reactions.

Question 33

What is a carboxylate ion?

Answer 33

An ion formed when a carboxylic acid molecule loses a proton.

Question 34

Are carboxylic acids weak or strong?

Answer 34

They are weak because the only slightly ionise when dissolved in water.

Question 35

How can an acyl chloride be made?

Answer 35

The reaction of a carboxylic acid with PCl₅ is the most common.

CH₃COOH (Ethanoic acid) + PCl₅ –> CH₃COCl + POCl₃ + HCl