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Dr stead chem > Optical isomerism > Flashcards

Optical isomerism Flashcards

1
Q

When does optical isomerism occur?

A

when a carbon atom has 4 different atoms or groups of atoms attached to it, resulting in an asymmetrical carbon/ chiral centre

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2
Q

What are enantiomers?

A

2 molecules that are non-superimposable isomers that are mirror images of each other

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3
Q

What is the difference between enantiomers?

A

physical properties the same- but when exposed to plane polarised light, the enantiomers rotate the plane in opposite directions the same amount

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4
Q

Which enantiomer rotates the plane CLOCKWISE?

A

dextrorotary

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5
Q

Which enantiomer rotates the plane ANTICLOCKWISE?

A

laevorotary

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6
Q

How is rotation of light measured?

A

using a polarimeter

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7
Q

What is a 50:50 mixture of equal amounts of enantiomer called?

A

a racemic mixture/ a racemate

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8
Q

Why is a racemic mixture optically inactive?

A

contains equal amounts of both enantiomers , so is optically inactive because the optical effects of each enantiomer cancel each other out

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9
Q

What happens when optically active molecules are made in nature?

A

only one enantiomer is formed

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10
Q

Why is only one enantiomer formed in nature?

A

enzymes are stereospecific, as they themselves are optically active, so only react either one enantiomer and produce one enantiomer

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11
Q

What are the two ways aldehydes and ketones undergo nucleophilic addition?

A

a) hydride ion (:H-) from NaBH4 in reduction in aqueous solution

b) the cyanide ion (:CN-) provided by KCN, followed by dilute acid

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12
Q

What is the name of NaBH4?

A

sodium tetrahydridoborate (III)

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13
Q

What are the products or nucleophilic additon of NaBH4 and aldehydes/ketones

A

aldehyde- primary alcohol
ketone- secondary alcohol

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14
Q

What happens between aldehydes/ketones and KCN?

A

undergo nucleophilic addition with dilute acid to form nitriles

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15
Q

When are chiral centres formed from ketones?

A

when the starting ketone is asymmetrical

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16
Q

Why is potassium cyanide dangerous?

A

highly toxic, HCN is also extremely toxic, also highly flammable

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17
Q

Why is the -CN useful?

A

increases length of carbon chain, easily converted into other functional groups once added

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18
Q

What does the reaction of aldehydes/ketones with acidified KCN produce?

A

a racemic mixture because it has a chiral centre

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19
Q

What is the science behind a racemic mixture?

A

because the carbonyl group is planar, so susceptible to attack by the :CN- nucleophile is equally likely from either side of the plane

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20
Q

What is a non-specific way of reducing aldehydes/ketones?

A

reduced with hydrogen in the presence of a nickel catalyst (catalytic hydrogenation) at 150°C

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21
Q

Why is it non-specific?

A

molecules containing both C=O and C=C will become completely desaturated

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22
Q

Why is the reduction by NaBH4 specific?

A

nucleophiles cannot attack alkenes (electron rich double bond repels nucleophiles), so they can target aldehydes/ketones

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23
Q

Why are carboxylic aids soluble up to bromine (included)?

A

can form hydrogen bonds with water

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24
Q

Why do carboxylic acids have a higher boiling point than comparable alkenes?

A

they can form hydrogen bonds with each other

25
Q

What do carboxylic acids react with?

A

metals, bases, metal carbonates/hydrogenocarbonates

26
Q

What is the equation for the reaction between carboxylic acids and metals?
Give the example of Mg and ethanoic acid

A

metal + acid -> salt + hydrogen
magnesium + ethanoic acid -> magnesium ethanoate + hydrogen
Mg + 2CH3COOH -> (CH3COO-)2 Mg2+ + H2

27
Q

What is the equation for the reaction between carboxylic acids and bases?
Give the example of sodium hydroxide and ethanoic acid

A

base + acid -> salt + water
sodium hydroxide + ethanoic acid -> sodium ethanoate + water
NaOH + CH3COOH -> CH3COO- Na+ + H2O

28
Q

What is the equation for the reaction and difference between carboxylic acids and metal carbonates/hydrogencarbonates?
Give the example of calcium carbonate and propanoic acid

A

metal carbonate + acid -> salt + water + carbon dioxide
calcium carbonate + propanoic acid -> calcium propanoate + water + carbon dioxide

CaCO3 + 2CH3CH2COOH -> (CH3CH2COO-)2 Ca2+ + H2O + CO2

difference is that for carbonates there is 2 moles of acid but for hydrogencarbonates there is only 1 mole

29
Q

How do you test for carboxylic acids?

A

-add sodium hydrogencarbonate
-bubbles of CO2 present if it is a carboxylic acid

30
Q

How do you make an ester through esterification?

A

carboxylic acid + alcohol ⇌ ester + water

31
Q

What is the condition for esterification?

A

concentrated H2SO4 acid catalyst

32
Q

Why is esterification not ideal for making esters?

A

it is a reversible reaction so has a low yield

33
Q

How do you name an ester?

A

first part is from the alcohol- change the name to -yl
methanol -> methyl
second part is from the carboxylic acid- change the name to -oate
ethanoic acid -> ethanoate

34
Q

What are esters used for?

A

perfumes, artificial food flavourings, plasticisers, solvents

35
Q

Esters and carboxylic acids are…?

A

functional group isomers

36
Q

What is the backward reaction of esterification?

A

ester hydrolysis- can happen in either acidic or alkaline solution

37
Q

Describe ester hydrolysis in acidic solution?

A

backward reaction, ester + water ⇌ carboxylic acid + alcohol, with a concentrated sulphuric acid catalyst

38
Q

Describe ester hydrolysis in alkaline solution?

A

boil with NaOH, quicker and leads to a greater yield due to a completed reaction
ester + sodium hydroxide –> sodium salt of carboxylic acid + alcohol

39
Q

What happens when you boil fats in NaOH?

A

makes soap

40
Q

Why does glycerol have a high viscosity?

A

forms hydrogen bonds, can also stop things drying out due to its ability to attract water

41
Q

What is the IUPAC name for glycerol?

A

propane-1,2,3-triol

42
Q

What are acyl chlorides and acid anhydrides?

A

derivatives of carboxylic acids

42
Q

How are acyl chlorides named?

A

after the carboxylic acid, ie- ethanoic acid becomes ethanoyl chloride

43
Q

Why are acyl chlorides and acid anhydrides so reactive?

A

both the O and the Cl are more electronegative than the carbon, so the carbon becomes electron deficient and readily attacked by nucleophiles

44
Q

How are acid anhydrides named?

A

after the carboxylic acid, ie- ethanoic acid becomes ethanoic anhydride

45
Q

What is acylation?

A

adding an acyl group to a molecule (R-C=O)

46
Q

Why are acyl chlorides and acid anhydrides more reactive than their corresponding carboxylic acid?

A

because they have electron deficient carbon atoms that are susceptible to nucleophilic attack

47
Q

What are the four nucleophilic addition-elimination reactions and their products?

A

1) water –> carboxylic acids + HCl
2) alcohols–> esters + HCl
3) 2ammonia–> amides + ammonium chloride
4) 2primary amines–> N-substituted amides + alcohol-ammonium chloride

48
Q

How do you test for acyl chlorides vs acid anhydrides?

A

-add water
-measure pH with pH meter
-pH is around 3 for acid anhydride as the carboxylic acid produced is a weak acid
-pH is around 0/1 for acyl chloride as when it reacts with water, HCl is also made which is a strong acid, in addition to the carboxylic acid

49
Q

Why use an acid anhydride over an acyl chloride?

A

-cheaper
-less corrosive
-less reactive and less susceptible to hydrolysis

50
Q

What happens in the nitration of benzene an its conditions?

A

step 1-making the electrophile, (+NO2)
step 2- electrophilic substitution

conditions- concentrated H2SO4 catalyst, 50C, as any higher than 50C multiple substitutions can occur

51
Q

What is the equation for making the nitronium ion?

A

HNO3+2H2SO4 –> +NO2 + H3O+ + 2HSO4-

52
Q

What can nitrated benzene derivatives be used for?

A

explosives

53
Q

How do you make soap?

A

triglycerides + sodium hydroxide –> (boil) SOAP + glycerol

54
Q

What are common uses of esters?

A

-perfumes
-plasticisers
-solvents

55
Q

What is one similarity and one difference between the acyl chlorides and acid anhydrides in acylation reactions?

A

both can provide the acyl group for the acylation of another molecule, but the reaction of the acid anhydrides are less vigorous

56
Q

Why are acid anhydride and acyl chlorides more reactive than carboxylic acids?

A

C-Cl and C=O bonds are more polar as Cl and O are more electronegative than carbon, so carbon atom is more reactive as it is more electron deficient

57
Q
A

Dr stead chem (21 decks)

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