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A Level / GCSE Chemisty > ⬛️ Topic 17 - Organics II > Flashcards

⬛️ Topic 17 - Organics II Flashcards

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1
Q

What are the observations when sodium/potassium dichromate is added to an aldehyde? What gives the colour?

A

Orange —> green (due to Cr3+ ions)
(Sodium dichromate is an oxidising agent [O] and an aldehyde will oxidise (to a carboxylic acid))

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2
Q

What are the observations when sodium/potassium dichromate is added to ketone?

A

No observations as sodium dichromate is an oxidising agent and a ketone will not oxidise

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3
Q

What are the observations when sodium/potassium dichromate is added to a primary/secondary alcohol?

A

Orange —> green
(As primary and secondary alcohols will oxidise)

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4
Q

What are the observations when Benedict’s solution is added to an aldehyde? What is Benedict’s solution made from, ie. what gives the colour?

A

Blue —> (yellow-green precipitate) —> red precipitate (if boiling continues)
(Benedict’s is an oxidising agent and an aldehyde will oxidise - the Benedict’s contains Cu2+ ions which reduce to Cu2O which is red)

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5
Q

What are the observations when Benedict’s solution is added to a ketone?

A

Nothing - a ketone does not oxidise

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6
Q

What are the observations when Tollens is added to an aldehyde? What is Tollens made from?

A

A silver lining (mirror) forms on the inside of the test tube (from a colourless solution)
(Tollens is an oxidising agent [O] and an aldehyde will oxidise - contains [Ag(NH3)2]+ which reduces to Ag atoms)

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7
Q

What are the observations when Tollens is added to a ketone?

A

Nothing - a ketone will not oxidise

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8
Q

What are the observations when Fehling’s is added to an aldehyde?

A

Blue —> red precipitate
(Fehling’s is an oxidising agent [O] and an aldehyde will oxidise)

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9
Q

What are the observations when Fehling’s solution is added to a ketone?

A

Nothing - a ketone will not oxidise

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10
Q

What is the qualitative test for C=O (a carbonyl group)? What is a positive result? How can you identify the specific carbonyl compound?

A

Reaction with Brady’s (2,4-dinitrophenylhydrazine) - an orange precipitate will form from a colourless solution
- the specific carbonyl compound can then be identified by the melting temperature of this precipitate (derivative)

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11
Q

How do you reduce an aldehyde back to a primary alcohol/ reduce a ketone back to a secondary alcohol?

A

React with lithium tetrahydridoaluminate (LiAlH4) in dry ether [H]

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12
Q

How do you test for the presence of H3C-C(=O)-R [a CH3 bonded to the C=O] or a molecule that has potential to make this (eg. H3C-C(OH)(H)-R which will oxidise to make a C=O)? What is a positive result?

A

Reaction with iodine in alkali (iodoform) - a yellow precipitate (CHI3 - iodoform) forms from a colourless solution, with a ‘hospital’/antiseptic smell

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13
Q

How do you make a hydroxynitrile (-OH and -CN group) from a carbonyl compound (compound with a C=O)? What is the mechanism called?

A

React with HCN in the presence of KCN (the HCN is very toxic so only a small amount used [and use in fume cupboard], the KCN increases the concentration of CN-)
NUCLEOPHILIC ADDITION - need to know mechanism
Conditions: Reflux
(Reaction with aldehydes/ asymmetrical ketones give rise to optical isomers)

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14
Q

Order, from lowest to highest, the boiling points of aldehydes/ketones, alkanes, carboxylic acids and alcohols. Give reasons.

A
  • alkanes - only weak intermolecular forces (London forces) as non-polar
  • aldehydes/ketones - intermolecular forces intermediate strength (permanent dipole-dipole attractions) as polar C=O group - [do not form INTERMOLECULAR hydrogen bonds as all H are joined to carbon atoms)
  • alcohols - strong intermolecular forces (hydrogen bonding) as very polar OH group
  • carboxylic acids - very high boiling point - strong intermolecular forces (hydrogen bonding) - (very efficient hydrogen bonding as three polar bonds?)
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15
Q

Order, from lowest to highest, the solubility in water (/polar solvents) of aldehydes/ketones, alkanes, carboxylic acids and alcohols. Give reasons.

A
  • alkanes - not soluble in water as non-polar and hydrogen bonds cannot form
  • aldehydes/ketones - (of shorter chain length) are soluble in water as they can from hydrogen bonds WITH THE WATER molecules [between C=O and water molecules]
  • alcohols - more soluble in water as have O-H so can form stronger hydrogen bonds with the water)
  • carboxylic acids - most soluble in water as they can form more hydrogen bonds with the water molecules (between the C=O and the water molecule and between the O-H and the water molecule)
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16
Q

What is produced when a carboxylic acid is reacted with a base? What do the products all have in common?

A

A carboxylate salt is formed - always soluble (eg. Lithium ethanoate, Potassium butanoate)
(Neutralisation)

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17
Q

How do you reduce a carboxylic acid? Reagent? What is the product?

A

Reduction to primary alcohols ONLY (not to aldehyde first as aldehydes are more easily reduced than carboxylic acids so the aldehyde produced during the reaction will be immediately reduced to a primary alcohol)
Reducing agent: lithium tetrahydridoaluminate (LiAlH4) - [H] - in dry ether
(Water as by-product)

18
Q

How do you halogenate a carboxylic acid? Reagents/conditions? Product? Observations?

A
  • Halogenation to acyl chlorides
  • Reagent: PCl5 (phosphorus (V) chloride)
  • Conditions: anhydrous conditions (both PCl5 and acyl chloride react with water) room temperature (vigorous reaction so no heat required)
  • HCl(g) is produced as a by-product so misty fumes
  • POCl3 is also produced as a by-product and has to be separated by fractional distillation
19
Q

What does the esterification of a carboxylic acid involve? Reagents/conditions? Products?
What does the esterification of an acyl chloride involve?

A
  • esterification to esters
  • reagent: an alcohol

From carboxylic acid: (in equilibrium - this is the reverse reaction of hydrolysis of esters)
- condition: small amount of an acid catalyst (often conc.H2SO4) + heat
- however even with a catalyst the reaction is slow and reversible
- (water as by-product)

From acyl chloride: (gives better yield as goes to completion rather than being in an equilibrium)
- condition: room temperature (acyl chlorides more reactive than carboxylic acids)
- (HCl(g) as by-product)

20
Q

How do you prepare a carboxylic acid by oxidation? Oxidation of what? Oxidising agent? Conditions? Method?

A
  • oxidation of a primary alcohol/ aldehyde
  • oxidising agent: acidified potassium dichromate [O] - K2Cr2O7 + H2SO4
  • conditions: heat under reflux
  • method: when oxidation complete (orange —> green), reaction mixture is fractionally distilled to obtain a pure sample of the carboxylic acid
21
Q

How do you prepare a carboxylic acid by acid/alkaline hydrolysis? Hydrolysis of what? Reagents/conditions? What happens? What extra needs doing for alkaline hydrolysis?

A
  • hydrolysis of a nitrile to make a carboxylic acid / hydrolysis of a hydroxynitrile to make a hydroxycarboxylic acid
  • conditions: heat under reflux
  • reagents: either a dilute acid (H+) or aqueous alkali (OH-) [and H2O]
  • the CN bond breaks - the carbon atom remains (no change in no. of carbons) and the nitrogen becomes either the ammonium ion/ammonia (respectively)
  • for alkaline hydrolysis, the product is a carboxylate ion - dilute acid (H+) is added to convert it to the carboxylic acid
22
Q

What is significant about reactions of acyl chlorides?

A
  • All reactions produce a different ‘main’ product, but they will ALL produce HCl(g) which presents as white fumes (as acidic this can also be tested for with damp litmus test)
  • (Acyl chlorides are also highly reactive due to the electron deficient carbon attached to the oxygen and chlorine - easy attack by a nucleophile [more reactive than carboxylic acids]), therefore all conditions are just room temperature
23
Q

How do you make a carboxylic acid from an acyl chloride?

A

Reagent: (cold) water
Conditions: room temperature

(Acyl chlorides react with water, so don’t dissolve - aren’t soluble)

24
Q

How do you make a primary amide from an acyl chloride? What might the by-product be?

A

Reagent: concentrated ammonia solution
Conditions: room temperature
(NH4Cl may be produced instead of HCl as the NH3 will react with the HCl)

25
Q

How do you make a secondary/tertiary amide (N-substituted amides) from an acyl chloride?

A

Reagent: primary amine / secondary amine (respectively)
Conditions: room temperature

26
Q

What are the two types of hydrolysis of esters?

A
  1. Hydrolysis in acidic solution (the acid is a catalyst)
    Ester + water <—> carboxylic acid + alcohol (REVERSIBLE)
    Conditions: acid catalyst (often H2SO4), warm (slow reaction)
    [exact opposite of esterification of a carboxylic acid]
  2. Hydrolysis in alkaline solution (the alkali is a reactant)
    Ester + alkali (OH-) —> carboxylate salt + alcohol (GOES TO COMPLETION)
    (Carboxylate salt can be converted into carboxylic acid by adding a dilute acid (H+))
27
Q

Explain the relative boiling point and solubility of esters

A

Relatively low boiling points and are insoluble in water - all hydrogen atoms in the molecules are attached to carbon atoms, so hydrogen bonding is not possible

28
Q

What type of polymerisation is the formation of polyesters, and why is this different to the other type?

A
  • Polyesters are formed by condensation polymerisation - a small molecule (H2O or HCl) is released as a by-product of the ester bond forming
    - two different functional groups are needed - a DIcarboxylic acid (or DIacylchloride) and a DIol - either on two different molecules, or a monomer with a COOH (or COCl) AND a OH
  • Other type is addition polymerisation (occurs with alkenes - or molecule with C=C), only one type of molecule needed (then just added to each other)
29
Q

Why are diacyl chlorides generally not used in place of dicarboxylic acids when making polyesters?

A

Although an ‘alcohol + acyl chloride —> ester’ will give a better yield (ie. The preferred reaction to make an ester), diacyl chlorides are not generally used to make polyesters (even though they are more reactive than carboxylic acids, so a catalyst not needed, and it is not a reversible reaction) as:
- the by-product of the reaction is HCl(g) which is a very toxic gas

30
Q

Why can polymers not be used in acidic conditions?

A

If the polymer is in prolonged contact with an acid, hydrolysis will occur

31
Q

What is an optical isomer?

A

A type of stereoisomer which occurs in molecules where a carbon atom has 4 different groups (therefore no double bonds) bonded to it (the chiral centre). The two optical isomers (also known as enantiomers) are non-superimposable (chiral) [meaning one cannot be put on top of the other and they match up - they are mirror images]

32
Q

What is optical activity? What characteristic do all optical isomers have?

A

The ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light
(Of the enantiomer, NOT the solution formed (as usually a racemic mixture so no rotation))

An optically active molecule has no planes of symmetry.

(Monochromatic light - ‘normal’ light - oscillates in every direction)
(Plane polarised light - oscillates in only one direction)

33
Q

What are the differences and similarities between two optical isomers (enantiomers) (L- and D-)

A

Similarities:
- have identical chemical + physical properties

Differences:
- effect on plane polarised light (direction of rotation)
- their reactions with other chiral molecules

34
Q

What is polarimetry, and what effect do the enantiomers have?

A

The method by which we can tell L- and D- isomers from each other (cannot tell difference by just looking at them) - plane polarised light is passed through solution of the chiral substance (which contains both enantiomers)

D-isomer (+) will rotate the light clockwise
L-isomer (-) will rotate the light anti-clockwise

35
Q

Can there be a chiral centre on a cyclic structure or a benzene ring?

A

There can be a chiral carbon in a cyclic structure but NOT a benzene ring

36
Q

If there are 2 chiral centres in a molecule, how many optical isomers are there?

A

4 optical isomers (as 4 combinations of +/- forms)

37
Q

What is a racemic mixture?

A

An equimolar mixture of enantiomers (ie. the same number of moles /concentrations of the D and L isomers). The racemic mixture will have no overall effect on rotation of plane polarised light as the rotations cancel each other out (a D and L isomer of same concentrations will rotate the light by equal and opposite amounts)

38
Q

What does it mean if there is no rotation of plane polarised light?

A

There are no optical isomers OR a racemic mixture is formed

39
Q

What are the two types of nucleophilic substitution mechanisms, and what’s the difference between them?

A

SN1: (s= substitution, n=nucleophilic)
- one species involved in rate determining step (so 2 steps overall - stable carbocation intermediate formed)
- produces 2 enantiomers - can form racemic mixture
- tertiary compounds

SN2:
- two species involved in rate determining step (so 1 step overall - [a transition step included])
- produces 1 enantiomer (as :OH- can only attack on opposite side to leaving group) - non-racemic mixture
- primary compounds

40
Q

What are two uses of esters?

A

Perfumes and flavourings

A Level / GCSE Chemisty (26 decks)

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