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Chemistry > TOPIC 17 : ORGANIC CHEMISTRY II > Flashcards

TOPIC 17 : ORGANIC CHEMISTRY II Flashcards

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1
Q

Explain what is meant by a chiral molecule and say how you would recognise an asymmetric carbon atom.

A

A chiral molecule is one that has no plane of symmetry. An asymmetric carbon atom is one that is attached to 4 different groups.

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2
Q

Why doesn’t a racemic mixture have any effect on the plane of polarisation of plane polarised light?

A

It is a 50/50 mixture of the two enantiomers. One of these is rotating the plane
of polarisation anti-clockwise, and the other is rotating it by exactly the same amount clockwise. They cancel each other out, so there is no overall effect on the light.

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3
Q

Define structural isomer

A

Same molecular formula different structural formulae

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4
Q

Define Chain isomers

A

Compounds with the same molecular formula but different structures of the carbon skeleton

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5
Q

Define Position isomers

A

Compounds with the same molecular formula but different structures due to different positions on the same functional group on the same carbon skeleton

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6
Q

Define functional group isomers

A

Compounds with the same molecular formula but with atom arranged to give different functional groups

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7
Q

What causes positional isomerism

A

Functional groups found at different positions on a chain

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8
Q

What are stereoisomers

A

Stereoisomers have the same structural formulae but have a different spatial arrangement of atoms

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9
Q

What causes chain isomerism

A

Different chain lengths due branching

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10
Q

What causes functional group isomerism

A

The position of atoms causes different functional groups eg = ketones and aldehydes

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11
Q

Explain how geometric isomerism arises

A

When there is a double bond between Carbons causing restricted rotation around the double bond and two different groups/atoms attached to both ends of the restricted double bond

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12
Q

What are the two types of stereoisomers

A

Geometrical isomerism ( E/Z) and optical isomerism

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13
Q

What is a Z isomer how do we determine if an isomer is a Z isomer

A

A Z isomer is when the priority groups on either side are on the same side

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14
Q

What is an E isomer how do we determine if an isomer is an E isomer

A

If the priority groups are on opposite sides then it is an E isomer

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15
Q

What is a priority group

A

The atom with the larger atomic proton number

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16
Q

When does optical isomerism occur

A

Occurs in carbon compounds with 4 different groups of atoms attached to a carbon

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17
Q

What shape are optical isomers

A

Tetrahedral

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18
Q

Define a racemic mixture

A

A racemic mixture is a mixture containing equal amounts of two enantiomers (optical isomers) that rotate plane polarised light in opposite directions by the same amount

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19
Q

What are enantiomers

A

Optical isomers

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20
Q

What is special about optical isomers

A

They have similar chemical and physical properties but rotate plane polarised light by the same amount in different directions

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21
Q

How is a racemate mixture formed use an example

A

Nucleophilic addition of HCN to aldehydes and unsymmetrical ketones from both sides forms a racemate

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22
Q

What is optical activity

A

When an optical isomer rotates plane polarised light

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23
Q

What is a chiral molecule

A

A carbon atom that has 4 different groups attached

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24
Q

Give an example of how optical activity provides evidence for SN1 mechanisms. DESCRIBE the mechanism for 2 bromo-butane reacting with a OH- ion

A

The Br breaks away from the haloalkane to form a carbocation intermediate

The OH- ion can attach from other side resulting in different enantiomers and a racemate forms

Because a racemate forms they will be no optical activity in the products proving SN1 mechanisms

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25
Q

Give an example of how optical activity provides evidence for SN2 mechanisms
Describe the mechanism for 2 bromo- butane with a OH - ion

A

Give an example of how optical activity provides evidence for SN2 mechanisms
Draw the mechanism for 2 bromo- butane with a OH - ion

In an SN2 mechanism there is no intermediate species formed

The OH- attacks the carbon and the bromine breaks off

If the reactant was chiral than during the reaction the opposite enantiomer would form

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26
Q

How else can we get a racemic mixture? Draw the mechanism

A

We can form a racemate by the electrophilic addition of HBr to an unsymmetrical alkene

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27
Q

What test would you use to distinguish between hexane and hex-1-ene ? Give the results of the test for each substance

A

Bromine water

Bromine water is an orange solution of bromine and it becomes colourless when an alkene is present as alkenes decolourise bromine water

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28
Q

What is meant by non-superimposable

A

Cannot be placed above one another and look the same

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29
Q

Define plane polarized monochromatic light

A

Light that only oscillates in one plane and is one colour

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30
Q

State how separate samples of 2 - enantiomers could be distinguished in a laboratory

A

Observe the directions they rotate plane polarised light

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31
Q

What is formed during a reaction undergoing SN1

A

Carbocation intermediate

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32
Q

What is involved during an SN2 mechanism

A

Two molecules are involved, the nucleophile attacks the carbon cation when the halogen breaks off

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33
Q

How can we separate samples of optical isomers be distinguished

A

Pass plane polarized light through the isomers and the isomers will rotate the plane polarized light in opposite directions.

34
Q

Why do alcohols have higher boiling points than alkanes

A

Alcohols have higher boiling points than an alkane with the same number of carbon atoms due to hydrogen bonding.

35
Q

What is formed from the oxidation of a primary alcohol

A

You get an aldehyde if you distill or a carboxylic acid if you reflux or oxidise the aldehyde

36
Q

What is formed from the oxidation of a secondary alcohol

A

ketone

37
Q

What is formed from the oxidation of a tertiary alcohol

A

They do not oxidise

38
Q

What reagent is used to oxidise the alcohols or aldehydes

A

Potassium dichromate (K2 Cr2 O7)

39
Q

What does the chromium species get reduced to and state the colour change

A

The orange dichromate ion (Cr2O72-) is reduced to the green Cr3+ ion

40
Q

How would you prepare to form an aldehyde from a primary alcohol

A

Add potassium dichromate solution K2Cr2O7 to excess alcohol and distill off the aldehyde as soon as it forms

41
Q

Describe what happens when a reaction mixture is refluxed and why it is necessary in this case for complete oxidation to ethanoic acid

A

A mixture of liquids are heated to boiling point for a long time

Vapour is formed which escapes from the liquid mixture and condenses and returns to the liquid mixture

Any ethanol and ethanal that evaporated can be oxidised

42
Q

What are the circumstances for us including numbers to show the position of the double bond in ketones

A

When ketones have 5 CARBONS OR MORE you need to add a number to show the position of the double bond eg pentan-2-one

43
Q

When is the prefix oxo used

A

It is used for compounds that contain a ketone group in addition to a carboxylic acid or aldehyde attached

44
Q

What reagents are used in oxidation and how can they easily be expressed

A

Potassium dichromate it can easily be expressed as [O]

45
Q

What reagents are used in reduction and how can they easily be expressed

A

LiAlH4 and it can easily be expressed as [H]

46
Q

Why must LIALH4 be dissolved in dry ether

A

LiALH4 reacts violently with water and alcohols and the dry ether prevents this.

47
Q

Describe the solubility of small carbonyls

A

Small carbonyls are soluble in water because they can form hydrogen bonds in water

48
Q

Describe the solubility of pure carbonyls

A

They cannot hydrogen bond but bonds instead by permanent dipole bonding

49
Q

What are the two other reagents that can oxidise an aldehyde into a carboxylic acid and what are the conditions,reaction and observations of each

A

Tollens Reagent=

Conditions= heat gently
Reaction and observation = aldehydes only are oxidised by tollens reagent into carboxylic acids and the silver ions are reduced into silver atoms and a silver mirror is produced,

Ketones result in no change

Fehling’s solution=

Conditions= heat gently

Reactions= aldehydes only are oxidised by fehling’s solution into a carboxylic acid and the copper ions are reduced to copper oxide

Observation= BLUE CU 2+ ions in solution change to a red precipitate of Cu2O. KETONES DO NOT REACT

50
Q

What test do we use to identify carbonyl compounds and describe the process

A

Brady’s reagent (2,4, DNP) - product is an orange precipitate

Rinse the carbonyl with brady’s reagent
Filter precipitate
Recrystallise the precipitate using maximum amount of hot ethanol
Measure the melting and boiling temperature and dry purified product
Refer to data to determine carbonyl

51
Q

A mechanism of the reaction between an aldehyde and hydrogen cyanide name what type of mechanism this is and condiitons

A

Nuclophilic addition - Conditions= Room temperature and pressure, HCN in presence of KCN

52
Q

Why is Hydrogen cyanide in the presence of KCN

A

The KCN increases the concentration of the CN- ion

53
Q

What is a nucleophile

A

Donates an electron pair, negatively charged and is attracted to areas of low electron density

54
Q

What carbonyls can react with iodine in the presence of sodium hydroxide

A

Only carbonyls with a methyl group next to the C=O bond can do this reaction. Ethanal is the only aldehyde that reacts. Methyl ketones mostly react

55
Q

Write the equation for ethanal + iodine sodium hydroxide and state the product formed

A

CH3COCH3 +3I2 +4NaOH > CHI3 + 3NaI + 3H2O

The product formed is CHI3 which is a yellow precipitate with an antiseptic smell

56
Q

What is the name of the salt formed from carboxylic acid and the name the salt from ethanoic acid

A

A carboxylate, ethanoic acid= ethanoate

57
Q

Describe the solubility of carboxylic acids in water

A

Small carboxylic acids can dissolve in water (up to butanoic acid) after this solubility decreases.

58
Q

Describe the acidity of carboxylic acids

A

They are weak acids in water and only partially dissociate

59
Q

What is a dimer

A

Two carboxylic acids bonded together by a covalent bond

60
Q

Explain how the ability of ethanoic acid to dimerise affects its boiling point

A

The boiling point will be higher as the molecule will be larger and contain twice the number of electrons meaning greater london forces and more energy is required to break them

61
Q

Suggest an explanation for why sodium ethanoate is a solid at room temperature while ethanoic acid is a liquid

A

Sodium ethanoate is an ionic lattice so will have strong electrostatic forces of attraction between oppositely charged ions. Lots of energy is required to overcome this attraction. Ethanoic acid has intermolecular forces between molecules which require less energy to overcome

62
Q

What are the three ways you can prepare a carboxylic acid

A

Full oxidation of primary alcohols, Oxidation of aldehydes, hydrolysis of nitriles

63
Q

Name the reagent, conditions and the reaction of the hydrolysis of nitriles

A

Reagent= Dilute HCL/sulfuric acid

Conditions= Heat under reflux

Reaction = Nitrile > Carboxylic acid

64
Q

What is formed when you react an acid + metal

A

Salt + hydrogen

65
Q

What is formed when you react an acid + alkali

A

Salt + water

66
Q

What is formed when you react an acid + carbonate

A

Carbon dioxide + salt + water

67
Q

What is the only carboxylic acid that can be oxidised by using oxidizing agents and what does it form

A

Methanoic acid is the only one as its structure is effectively an aldehyde group it forms carbonic acid which decomposes to carbon dioxide

68
Q

Describe the reaction between carboxylic acids and phosphorus chloride and describe the conditions and observations and write an equation for the reaction of ethanoic acid reacting with phosphorus chloride

A

Reaction = Carboxylic acid > acyl chloride

Conditions= Room temperature

Observations= Misty fumes of HCL

Equation= CH3COOH + PCL5 > POCL3 + HCL+ CH3COCL

69
Q

What is formed when you react a carboxylic acid with alcohol and what is special about this reaction and the conditions

A

Carboxylic acid + alcohol > salt + water

It requires a strong acid catalyst (H2SO4), the reaction is slow and needs to be heated under reflux it is also reversible

70
Q

How do you name an ester

A

The part ending in yl comes from the alcohol that formed it

The part ending in anoate comes from the carboxylic acid

71
Q

How can esters be hydrolysed

A

They can be hydrolysed either by heating under reflux with acid or NaOH

72
Q

Name one way an ester can be hydrolysed and state the conditions, reagents and a reaction equation using the ester ethyl propanoate

A

Heating under reflux with dilute NaOH

Ethyl propanoate + NaOH > sodium propanoate + ethanol

This reaction is not reversible and goes to completion

73
Q

Name another way an ester can be hydrolysed and state the conditions, reagents and a reaction equation using the ester ethyl propanoate

A

Heating under reflux with dilute HCL acid and heat under reflux

Ethyl propanoate + water&raquo_space; Propanoic acid + ethanol

74
Q

What are the conditions and reagents and products for reacting an acyl chloride with water and state the observation

A 
Products= Carboxylic acid 
Reagent = water 
Conditions= room temperature 
Observation= Steamy white fumes of HCL
75
Q

What are the reagents,conditions and products for reacting an acyl chloride with alcohol and state the observation

A 
Products= Ester 
Reagent = alcohol
Conditions= room temperature 
Observation= Steamy white fumes of HCL
76
Q

What are the reagents,conditions and products for reacting an acyl chloride with ammonia and state the observation

A 
Products= primary amide 
Reagent = ammonia 
Conditions= room temperature 
Observation= white smoke of NH4Cl is give off
77
Q

What are the reagents,conditions and products for reacting an acyl chloride with primary amines

A 
Products= Secondary amide 
Reagent = Primary amine 
Conditions= room temperature 
Observation= Steamy white fumes of HCL
78
Q

What are the two types of polymerisation

A

Addition polymerisation and condensation polymerisation

79
Q

How to produce polyesters

A
  1. diol + diacarboxylic acid > Water + polyester

2. diol + diacyl dichloride > HCL + polyester

80
Q

Why Is chloroethanoic acid more acidic than ethanoic acid

A

Chlorine is more electronegative and therefore withdraws the electrons from COOH decreasing COOHs electron density and less negative so it becomes more stable and therefore chloroethanoic acid is more acidic

81
Q

Why does increasing chain length of a carboxylic acid mean it is less acidic

A

Increasing the chain length increases electron releasing alkyl groups attached which make the COOH more negative and less stable so it is weaker

82
Q

What can polyesters be hydrolysed by

A
  1. Acid or alkali
  2. With HCL a polyetser will be hydrolysed and spilt up into the original dicarboxylic acid and diol
  3. With NaOH a polyester will be hydrolysed and split up into the diol and dicarboxylic acid salt

Chemistry (20 decks)

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