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Chemistry A Level > Organic Chemistry II > Flashcards

Organic Chemistry II Flashcards

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1
Q

Chirality and Optical Isomerism

A

Chirality occurs in carbon compounds with 4 different groups attached to the carbon

Chiral molecules have similar physical and chemical properties but rotate plane polarised light in different directions

Optical isomerism results from chiral centres in a molecule - when the carbon has 4 different groups attached to the centre

The compounds have non-superimposable mirror images so they cannot match when directly on top of eachother

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2
Q

Optical Activity

A

An optically active substance will rotate plane-polarised light when a beam of monochromatic light is shone through a solution of an optically active substance

This is the only way to distinguish between two pure samples of enantiomers to find out the direction of rotation

Dextrorotatory (D) enantiomer rotates clockwise (+)

Laevorotatory (L) enantiomer rotates anticlockwise (-)

  1. Light source produces light vibrating in all directions
  2. Polarising filter only allows through light vibrating in one plane
  3. Plane polarised light passes through optically active sample
  4. Plane polarised light is rotated
  5. Analysing filter turned so that light reaches maximum
  6. Direction of rotation is measured coming toward observer
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3
Q

Racemic Mixture

A

A racemic mixture contains a 50:50 mixture of two enantiomers

Enantiomers are two molecules that are optical isomers of each other

A racemic mixture will not rotate plane-polarised light as the two opposing rotations will cancel each other

They are formed when a trigonal planar reactant or intermediate is attacked from both sides so there is an equal chance of forming each enantiomer

Reaction Mechanism

SN1

During hydrolysis by SN1 mechanism, a planar carbocation intermediate is formed and the OH- ions can approach from either side with equal probability
If the reactant is one isomer of a chiral halogenoalkane, the product will be a racemic mixture of the two optical isomers of the alcohol

SN2

During hydrolysis by SN2 mechanism, a chiral carbon atom undergoes inversion of its stereostructure so if the reactant is an optically active halogenoalkane, it will form an optically active alcohol

Halogenoalkane → alcohol

The different enantiomers of a chiral halogenoalkane, produce different enantiomers of the alcohol

  • The nucleophile can only attack from one side
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4
Q

Aldehydes

A

Aldehydes are formed through the oxidation of primary alcohols

  • Form permanent dipole interactions
  • Soluble in water
  • Polar due to the difference in electronegativity
  • Attacked by nutrophiles
  • Used in preservatives, flavourings and perfumes
  • There is no hydrogen bonding between aldehydes themselves
  • In large non-polar hydrocarbon groups, the collective london forces require lots of energy to overcome
  • Short chain aldehydes and ketones are water soluble, whereas longer carbon chains are less soluble
  • The oxygen on the carbonyl group can hydrogen bond with -OH group in water
  • The carbonyl group causes dipole dipole interactions and london forces between molecules
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5
Q

Ketones

A

Ketones are formed through the oxidation of secondary alcohols

Reagents: hot acidified potassium dichromate

Not easily oxidised further

  • Contains a carbonyl group
  • Forms permanent dipole interactions
  • Soluble in water
  • Used in nail polish removers, embalming fluids, perfumes and pesticides
  • There is no hydrogen bonding between ketones themselves
  • In large non-polar hydrocarbon groups, the collective london forces require lots of energy to overcome
  • Short chain aldehydes and ketones are water soluble, whereas longer carbon chains are less soluble
  • The oxygen on the carbonyl group can hydrogen bond with -OH group in water
  • The carbonyl group causes dipole dipole interactions and london forces between molecules
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6
Q

Reaction with Fehlings solution

A

Fehling’s solution

  1. Put 1cm3 Fehling’s solution A and then add Fehling’s solution 2 until the precipitate formed dissolves
  2. Add 7 drops of the aldehyde/ketone
  3. Shake gently and place in a beaker of boiling water until not further colour change

Aldehyde

  • Blue ppt on heating
  • Red ppt forms
  • Oxidation occurs

(Cu2+ → Cu1+)
Cu2+ + e- → Cu1+

Ketone

  • no change
  • cannot be oxidised
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7
Q

Reaction with Potassium Dichromate

A

Potassium dichromate

  1. Put 5 drops of the aldehyde/ketone in a test tube
  2. Add 2 drops of 0.1M potassium dichromate (VI) solution
  3. Add 10 drops 1M sulphuric acid
  4. Shake gently and put in a beaker of warm water

Aldehyde

  • Turns blue/green
  • Can be oxidised
  • Chromate is reduced to chromium

Ketone

  • no change
  • cannot be oxidised
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8
Q

Reaction with Tollens Reagent

A

Tollens reagent

  1. Put 1cm3 0.05M silver nitrate into a clean test tube
  2. Add 3-4 drops 2M sodium hydroxide solution
  3. Add 2M ammonia solution drop by drop until the precipitate nearly dissolves
  4. Add 1-2 drops of the aldehyde/ketone and place in a beaker of warm water

Aldehyde

  • Silver mirror forms
  • Silver ions reduced to silver atoms

Ketone

  • no reaction when heated
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9
Q

Reaction with LiAlH4 in Dry Ether (Reduction)

A

Aldehyde/ketone → Alcohol

LiAlH4 is a powerful reducing agent - it releases H- ions which act as nucleophiles and attack the partially positive carbonyl carbon atom

Must be carried out in the absence of water as it would react very quickly with water

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10
Q

Reaction with Iodine in the presence of Alkali

A

Shows the presence of a methyl group next to a carbonyl group

  1. Place 5 drops aldehyde/ketone in a test tube
  2. Add 1cm3 iodine solution (10% in KI)
  3. Add 2M sodium hydroxide drop by drop until the colour of the iodine disappears

Aldehyde

  • ethenal is the only aldehyde that reacts

Ketone

  • straw coloured precipitate of triiodomethane
  • 4
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11
Q

Reaction with 2,4-DNPH

A

Test to show presnce of a Carbonyl group

  1. Put 1-2 drops of aldehyde/ketone
  2. Add 2,4-DNPH

Aldehyde:

  • Colourless → orange
  • Ppt forms when H2SO4 is added

Ketone:

  • Colourless → orange
  • Orange ppt forms

Condensation/addition-elimination reaction

Orange crystalline solids formed have well defined melting points which are useful for identifying the carbonyl compound

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12
Q

Reaction with Cyanide Ions

A

Aldehyde/ketone → Hydroxynitrile

Reagents: HCN in the presence of KCN

Conditions: Room temperature and pressure

Mechanism: nucleophilic addition

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Chemistry A Level (10 decks)

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