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Chemistry Edexcel A Level > Topic 17: Organic II > Flashcards

Topic 17: Organic II Flashcards

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1
Q

What is optical isomerism

A

Isomers that contain asymmetric centres which form stereoisomers that differ in their effect on plane polarised light

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2
Q

What is a chiral carbon

A

A carbon in a compound which has 4 different groups attached to

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3
Q

What are enantiomers

A

They are mirror images of each other and are non-superimposable

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4
Q

What are some general properties of enantiomers

A
  • same boiling point, same solubility
    -differ in how they react with other molecules
    -differ in effect on plane-polarised light
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5
Q

How do enantiomers ‘change’ plane-polarised light

A

An enantiomer will rotate the plane-polarised light either clockwise or anti-clockwise

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6
Q

What is it called when there is no rotation of plane-polarised light

A

A racemic mixture/ racemate (50/50 mixture of two enantiomers)

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7
Q

What mechanism can form a racemic mixture

A

Sn1 reaction

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8
Q

Why can a racemic mixture be formed by an Sn1 reaction

A

-the C+ intermediate is Trigonal planar shape
-allows the Nu- to attack from either side
-this allows for the creation of 2 enantiomers in a racemic mixture
-test by seeing if there is a rotation of plane-polarised light

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9
Q

Why do large aldehydes and ketones (4+ C atoms) become more immiscible with water

A

-aldehydes and ketones have permanent dipole-dipole interactions between each other
-short aldehydes and ketones can form hydrogen bonds with H2O molecule, (oxygen from carbonyl and H from H2O) - these H bonds are similar in strength to the ones in water, so they will mix
-longer aldehydes + ketones will have more LFs, but smaller amount of H bonding, as chain increases. LFs are very weak than the H bonds in water, so they don’t mix

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10
Q

What are the conditions, reagents and product of the reduction of aldehydes + ketones

A

Mechanism: nucleophilic addition
Nucleophile: H-
Reagent: LiAlH4 (in dry ether) STRONG. Or NaBH4 MILDER.
Product: alcohol

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11
Q

What are the conditions, reagents and product of the cyanohydrin formation

A

Mechanism: Nucleophilic addition
Nucleophile: CN-
Reagent: KCN(aq), H+ cat.
Product: hydroxynitrile (OH and CN groups)
(Way of increasing carbon lengthening)

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12
Q

Draw the mechanism for the cyanohydrin formation reaction by nucleophilic addition

A

CN- attacks carbonyl carbon (on aldehyde or ketone), oxygen accepts electrons from one of the bonds in the double bond becoming negatively charged. electron takes a hydrogen ions (attacks) to form hydroxyl group.

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13
Q

What is the Grignard reaction and what do you need for it

A

-increases carbon chain length.
STEP 1: form Grignard reagent:
Halogenoalkane + magnesium (dry ether + heat) -> Grignard reagent
STEP 2: reaction with a carbonyl

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14
Q

What is needed for Iodoform reaction + what shows a positive test

A

-test for a methyl ketone
Reagent: I2 + NaOH
Product: Iodoform, if methyl ketone present (yellow precipitate and antiseptic smell)

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15
Q

What is needed for Tollen’s reaction + what shows a positive test

A

-test for aldehydes
Reagent: ammoniacal silver nitrate (tollen’s solution)
Observation: silver mirror as aldehyde is oxidised and Ag+ reduced to Ag(s)

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16
Q

What is needed for Fehling’s reaction + what shows a positive test

A

-test for aldehydes
Reagent: copper (II) sulphate + alkaline potassium sodium tartrate (FEHLING’S SOLUTION)
Observation: colour change from blue to brick red
(Cu2+ reduced to Cu+)

17
Q

What is needed for 2, 4-DNP reaction + what shows a positive test

A

-test for aldehydes and ketones
Reagent: 2, 4-DNP
Observation: orange solution -> orange precipitate
-precipitate can then be filtered and recrystallised and melting point measured. Melting point of 2,4-DNP derivative can be used to identify certain aldehyde/ ketone.

18
Q

How can you prepare a carboxylic acid

A

1) Oxidation of primary alcohol or aldehyde (K2Cr2O7 + H2SO4 (cat.)), heat under reflux, orange -> green
2) Hydrolysis of nitrile (C≡N)
Reagents: water + H+ catalyst (HCl), heat

19
Q

What are some properties of carboxylic acids

A

-they are weak acids: only dissociate a small amount in solution
-high boiling point (can form 2 hydrogen bonds per molecule)

20
Q

Why does ethanoic acid have the highest boiling point compared to butane, propanal and propan-1-ol

A

Ethanoic acid has London forces, permanent dipole-dipole interactions and can form 2 hydrogen bonds per molecule
-> it can create a dimer, which also increases the size of the London forces

21
Q

What are the conditions and reagents for reduction of carboxylic acid to primary alcohol

A

LiAlH4 (dry ether), followed by acid work-up

22
Q

What are the conditions and reagents for making esters from carboxylic acids

A

add alcohol, conc. H2SO4, heat under reflux
-this is a reversible reaction, low yield

23
Q

What are the conditions and reagents for making an acyl chloride from a carboxylic acid

A

1) Using PCl5 (MUST BE DRY), also produces POCl3 and HCl
2) Using SOCl2 (BETTER), by-products of SO2 and HCl can be easily removed

24
Q

What are the conditions and reagents for making a carboxylic acid from an acyl chloride

A

Add H2O

25
Q

What are the conditions and reagents for making an ester from an acyl chloride

A

Add alcohol, room temp (acyl chloride very reactive)
+ pyridine to neutralise the HCl formed

26
Q

What are the conditions and reagents for making an amide from a acyl chloride

A

Concentrated NH3, and NaOH to neutralise the HCl formed

27
Q

What are the conditions and reagents for making a substituted amide from an acyl chloride

A

Add an amine

28
Q

What are the conditions and reagents for making a carboxylate salt from a carboxylic acid

A

1) + metal (MASH)
2) + base

29
Q

What are the conditions, reagents and product for acid hydrolysis of esters

A

Hot aqueous acid (HCl)
Reversible - low yield, always in equilibrium

30
Q

What are the conditions, reagents and product for base hydrolysis of esters

A

Hot aqueous NaOH/ KOH, heat
One way reaction - high yield

31
Q

Why are acyl chlorides more reactive than carboxylic acids

A

In a carboxylic acid: very high electron density, carbon is less electrophilic and Nucleophiles don’t want to attack.
In acyl chlorides: C-Cl bond long enough and electrons density low enough around C atom, making it more electrophilic, so nucleophiles readily attack (only room temperature needed)

32
Q

What process forms polymers of carboxylic acids

A

Condensation polymerisation

33
Q

Properties of condensation polymers

A

Ester bond can be easily hydrolysed - biodegradable

34
Q

What are the conditions for alkali hydrolysis of esters

A

Hot aqueous NaOH
Irriversible reaction (two steps: hydrolysis to form salt + alcohol, acidification of salt to make carboxylic acid)

35
Q

What is the connection between optical activity and the following mechanisms:
SN1, SN2, Nucleophilic Addition

A

SN1; no optical activity as carbocation intermediate is trigonal planar: equal probability of attack from above or below the plane of the molecule
SN2; If an enantiopure reactant is used, an enantiopure product is formed due to steric hindrance preventing attack from both sides - i.e, must attack from opposite side of carbon to the halogen
NA; Carbonyl carbon is trigonal planar and therefore there is equal probability of attack from above or below the plane of the molecule

Chemistry Edexcel A Level (18 decks)

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