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AQA chemistry A level notes Diana Tut > The carbonyl group (A2) > Flashcards

The carbonyl group (A2) Flashcards

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1
Q

Define functional group [1]

A

A group of atoms or an atom responsible for the characteristic reactions of a molecule

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2
Q

Describe the features of the carbonyl group [1]

A
  • -C=O
  • C=O is polar (delta + carbon and delta - O) so the C delta + is susceptible to nucleophilic attack
  • C=O bond is unsaturated so can be involved in addition reaction (the pi bond can be broken and species added across it)
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3
Q

Describe the solubility of aldehydes/ketones in water

A
  • Aldehydes/ketones with a relatively small Mr are soluble as they can form H bonds between the LP of e- on the delta- O of the aldehyde and the delta+ H of the H2O.
  • Aldehydes/ketones with large Mr are insoluble because they have very long chain so the strength of the many VDW forces overcomes the strength of the few H bonds formed with the H2O, preventing the molecules from being separated
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4
Q

Place these molecules in order of ascending BP (assuming their Mr is similar): aldehydes, ketones, esters, carboxylic acids, alkanes, amines, alcohols and amides

A

alkanes (VDW forces only) < aldehyde/ketones/esters (dipole-dipole forces) < alcohols/amines (H bonds) < carboxylic acids/amides (H bonds and dipole-dipole forces)

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5
Q

How do you synthesise Aldehydes? include the chemicals and conditions required [2]

A
  1. Incomplete oxidation of primary alcohol Conditions: warm with acidified K2Cr2O7 and then distil off products immediately
  2. Reduction of carboxylic acids using aqueous NaBH4. Condition: heat under reflux with NaBH4 in aqueous ethanol then acidify using dilute H2SO4
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6
Q

Describe the Tollens reagent test

A
  • Tollens’ reagent is a mild oxidising agent containing the complex [Ag(NH3)2] +. It is formed by adding NaOH to AgNO3 solution to from a brown ppt of Ag2O. NH3 is added dropwise until the brown ppt dissolves to form a colourless solution (ammoniacal silver nitrate)
  • When a sample of aldehyde is warmed with Tollens’ reagent, a silver mirror forms on the test tube walls as the Ag+ ions are reduced to Ag
  • reduction equation = Ag+ + e- → Ag
  • ketones do not react as they are not easily oxidised so the solution remains colourless
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7
Q

Describe the Fehling’s solution test

A
  • Fehling’s solution is a mild oxidising agent that is blue and contains complex Cu2+ ions in alkaline solution
  • when you warm a sample of aldehyde in fehling’s solutions, the Cu2+ are reduced to Cu+, producing an orange-red ppt solution of Cu2O
    • reduction equation = Cu2+ + e- → Cu+
  • ketones do not react as they are not easily oxidised so the solutions remains blue
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8
Q

Describe the reduction of aldehydes/ketones

A
  • Aldehydes can be reduced to primary alcohols and ketones can be reduced to secondary alcohols, both via nucleophilic substitution
  • RCHO + 2[H] → ROH
  • RCOR + 2[H] → ROHR
  • :H- (from reducing agent) acts as a nucleophile then a H+ (from the acid) acts as a base after the C=O is broken
  • Conditions- heat under reflux with NaBH4 in aqueous ethanol followed by acidifying with dilute H2SO4
  • 2 moles of reducing agent [H] are needed
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9
Q

Why is NaBH4 a particularly useful reducing agent?

A

it is less reactive than other reducing agents so can be used to reduce a single carbonyl group in a molecule containing many functional groups to produce a certain chemical

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10
Q

Describe the reaction of aldehydes/ketones with hydrogen cyanide (HCN)

A
  • HCN is formed by adding dilute acid to KCN
  • aldehydes/ketones react with HCN to form 2-hydroxynitriles via nucleophilic substitution
  • a racemic mixture can be formed as the C=O bond is planar so the C delta+ has equal probability of being attacked from either side by the :CN nucleophile
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11
Q

What is the suffix for the ester functional group?

A

-anoate

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12
Q

Name this compound

A

Propyl methanoate

method:

split the ester into 2 - the part containing the carbonyl group (coming from the carboxylic acid) and the part containing the C-O bond (coming from the alcohol)

as the part coming from the carboxylic acid has higher priority, you name the compound as an -anoate with a alkyl branch

  • the alkyl branch has 3 carbon atoms so is called propyl
  • the left hand side has 1 carbon atoms so is called methanoate
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13
Q

Draw the functional group for acyl chlorides and describe the naming rule

A

(number of carbon atom prefic) plus -anoyl chloride

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14
Q

Draw the functional group of acid anhydrides

A
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15
Q

Name this compound

A

Ethanoic propanoic anhydride

name each component as a carboxylic acid in alphabetic order

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16
Q

Draw the functional group for amides and describe the naming rule

A

(prefix coming from the number of C atoms attached to C=O group) plus -anamide

atoms attached to N are added as a branch (N-alkyl group)

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17
Q

Draw the functional group for carboxylate salts

A

group 1 metal name plus (prefix from number of C atoms attached to C=O group) -anoate

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18
Q

name the amine functional group as a prefix

A

amino

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19
Q

draw the functional group for alkyl ammonium salts

A

the H atoms may be substituted by other alkyl groups

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20
Q

Name this compound

A

But-3-en-1-ol

This is an unsaturated alcohol

-OH takes priority over -C=C-

ene becomes en

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21
Q

Name this compound

A

But-2-enoic acid

the -ene and -anoic suffixes are combined to give -enoic

the carboxyl group has higher priority over the C=C bond so the number refers to the position of the C=C bond

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22
Q

Name the pairs homologous series that are functional group isomers of each other

A
  • ketones and aldehyde
  • alkenes and cycloalkanes
  • alcohols and ethers
  • esters and carboxylic acids
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23
Q

What are optical isomers

A
  • They are steroisomers which have non-superimposable mirror images
  • contain a chiral carbon centre
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24
Q

Describe the 2 types of optical isomers

A
  • as optical isomers have identical chemical and physical properties, their optical activity is used to distinguish the 2 enantiomers apart
    • enantiomers rotate the plane of polarised light clockwise
    • enantiomers rotate the plane of polarised light anticlockwise
25
Q

Define racemic mixture

A
  • equimolar mixture of + and - enantiomers
  • overall optically inactive (equal rotation both clockwise and anticlockwise so rotation cancels out)
26
Q

How do you separate the different enantiomers in a racemic mixture?

A
  • react the racemic mixture with a chiral reagent so that the products have different physical properties
  • alternatively use chromatography
27
Q

How can you tell if a mechanism will produce a racemic mixture or a single enantiomer?

A
  • identify the bond shape of the bond involved in the addition/substitution part of the mechanism
  • if it is symmetrical, a racemic mixture is produce
  • if it is asymmetrical, a single enantiomer is produced
28
Q

Explain the term “homologous series” [3]

A

A family of carbon containining compounds that have:

  • the same general formula
  • similar chemical properties
  • a gradation in physical properties
  • same functional group

Each successive member differs by a -CH2 group

29
Q

Define structural formula

A

Shows the arrangement of the atoms within a formula but does not show the bonds

30
Q

name this compound

A

N,N - dimethyl methanamide

1 C atom in the C=O part of the compound so methanamide

2 methyl groups attached to the N atom so N written twice followed by dimethyl

31
Q

describe the 4 ways carboxylic acids can be synthesised

A
  • complete oxidation of a primary alcohol
    • conditions: hot reflux with excess K2Cr2O7 and conc H2SO<span>4</span>
  • further oxidation of an aldehyde
    • conditions : hot relfux with excess K2Cr2O7 and conc H2SO<span>4</span>
  • hydrolysis of esters
    • general reaction :
    • ester + H2O ⇌ Carboxylic acid + alcohol
    • condition: conc H2SO4 catalyst and heat under reflux
    • disadvantage: reversible reactions never produce a 100% yield of products
  • hydrolysis of acid anhydrides
    • occurs at room temp
    • slow reaction
    • (RCO)2O + H2O → 2RCOOH
32
Q

Describe the acidity of carboxylic acids and give the equation of the reaction of carboxylic acids with water

A

weak acids as only partly dissociate into ions in water

the bigger the carboxylic acid, the weaker its acidity as bigger R groups repel the e- pairs towards the C 𝛿+ making it less likely for dissociation to occur

if electronegative atoms appear in the carboxylic acid however, the acidity is stronger as electronegative atoms will atract the e- pair towards themselves, making dissociation into ions more likely

33
Q

describe the common reactions and condititons of carboxylic acids

A
  • reaction with a hydroxide produces a sodium salt and water
    • R-COOH + NaOH → RCOONa + H2O
    • this is a non-reversible reaction as NaOH is a strong base
  • reaction with ammonia produces an ammonium salt
    • R-COOH + NH3 ⇌ R-COONH4
    • reversible reaction as ammonia is a weak base
  • reaction with a metal produces a salt and hydrogen
    • R-COOH + Mg → (RCOO)2Mg + ½H2
    • observation : effervescence and metal used up to give a colourless solution
  • esterification:
    • R-COOH + R-OH ⇌ R-COO-R + H2O
    • conditions: heat + conc H2SO4
  • dehydration reaction
    • 2 RCOOH → R(CO)2O
    • dehydrating agent: P2O5
34
Q

Give the reaction for the synthesis of acyl chlorides

A

R-COOH + PCl5 → R-COCl + PCl3O + HCl

35
Q

give the reaction for the synthesis of acid anhydrides

A
  • acid anhydrides are formed via the dehydration of two carboxylic acids
  • P2O5 acts as a dehydrating agent
  • a molecule of water is released
36
Q

what are fatty acids?

A

long chain carboxylic acids

37
Q

describe the ways to synthesise esters

A
  • esterification
    • R-COOH + R-OH ⇌ R-COO-R
    • conditions: heat and conc H2SO4 catalyst
  • triglyceride formation - triglycerides are triesters
    • 3 fatty acids + glycerol (propane-1,2,3-triol) → triglyceride + 3H2O
    • conditions : conc H2SO4 and heat under refluc
  • acylation of carboxylic acid derivatives
    • Acyl chloride + alcohol → ester + HCl
    • anhydrous conditions needed
    • Acid anhydride + alcohol → ester + carboxylic acid
38
Q

give the common reactions of esters

A
  • hydrolysis of esters in the presence of acid catalyst
    • R-COO-R + H2O ⇌ R-COOH + R-OH
  • hydrolysis with NaOH
    • this is also called saponification
    • R-COO-R + NaOH → RCOONa + R1-OH
    • conditions: heat until boiling point
  • hydrolysis of triglycerides with NaOH is used to make biodiesel
    • Triglyceride + NaOH → Sodium salt of fatty acid + Glycerol
    • triglycerides are found in cooking oil
39
Q

Describe the test for carboxylic acids

A
  • react with carbonate e.g NaHCO3
  • observation = effervescence (also turns colourless limewater cloudy)
40
Q

Define acylation

A
  • the process of replacing a H atom in a molecule by an Acyl group
  • it is carried out using either acyl chlorides or acid anhydrides
  • the organic product is always the same whether acyl chlorides or acid anhydrides are used, only the other byproducts differ
41
Q

why does heating under reflux ensure complete oxidation of primary alcohols?

A
  • vapours of intermediate aldehyde are condensed and fall back into the flask where they can be further oxidised by the excess oxidising agent
42
Q

how do you separate esters from the mixture of products formed in esterification reactions?

A

esters are volatile so have lower boiling points than the other products(as they don’t form H bonds while the other products do)

therefore they can be separated easily using simple distillation

43
Q

describe the properties of esters

A
  • similar BP to aldedhydes and ketones (as they all form dipole-dipole intermolecular forces)
  • solubility decreases as carbon chain length increases
    • small esters can form H bonds with water
    • in long-chain esters, the large hydrocarbon chain gets in the way of H bond formation with the water
  • volatile
44
Q

explain how soap can clean dirt

A
  • the carboxylate salt has a hydrophobic hydrocarbon tail and a hydrophilic head (containing the COONa group)
  • dirt is mostly oil that will not come off with water because oil is hydrophobic so cannot dissolve in water
  • when soap molecules comes into contact with water, the hydrophobic tails bind to the oils
  • micelles droplets form with the hydrophilic heads facing the water and the hydrophobic tails facing inwards.
  • the dirt is trapped in the micelles which are soluble in water so are washed away with the water
45
Q

why does ethanoic anhydride smell strongly of vinegar?

A
  • it reacts with water vapour in air to reform ethanoic acid which smells of vinegar
46
Q

describe the properties of acid anhydrides

A
  • insoluble in water
  • similar BP to acyl chlorides and esters with the same number of carbons
47
Q

how are acid anhydrides synthesised?

A

dehydration of 2 carboxylic acids (either the same or different) using P2O5

48
Q

give the common reactions of acid anhydrides

include the conditions/observations

A
  • reaction with water hydrolyses acid anhydrides back to the carboxylic acids they were made from
    • (RCO)2O + H2O → 2 RCOOH
    • happens slowly at room temp
  • in acylation reactions:
    • reaction with alcohol makes an ester and carboxylic acid
      • (RCO)2O + R1OH → RCOOR1 + RCOOH
    • reaction with ammonia makes a 1º amide and carboxylic acid
      • (RCO)2O + NH3 → RCONH2 + RCOOH (or RCOONH4 )
    • reaction with a 1º amide makes a 2º amide
      • (RCO)2O + R1NH2 → RCONHR1 + RCOOH (or RCOOR1NH3)
49
Q

describe the properties of acyl chlorides

A
  • insoluble in water because they react vigorously with water so its no possible to get a simple aqueous solution of acyl chlorides
  • similar BP to acid anhydrides and esters containing same carbon number
50
Q

give the common reactions of acyl chlorides

A
  • reaction with water is very vigorous and unsafe as it releases toxic steamy fumes of HCl . carboxylic acid is produced
    • RCOCl + H2O → RCOOH + HCl
  • acylation reactions
    • reaction with alcohol makes an ester
      • RCOCl + R1OH → RCOOR1 + HCl
      • room temp
      • anhydrous conditions required
    • reaction with ammonia produces a 1º amide and an ammonium salt because the HCl acid reacts with the ammonia base
      • RCOCl + 2NH3 → RCONH2 + NH4Cl
    • reaction with 1º amide produces a 2º amide and a salt
      • ROCl + 2R1NH2 → RCONHR1 + R1NH3
51
Q

what are the advantages of using acid anhydrides instead of acyl chlorides in acylation reactions?

A
  • cheaper to make
  • produce safer products ( do not release toxic HCl when in contact with water)
  • less corrosive
  • less vulnerable to hydrolysis
52
Q

give the general mechanism for acylation reactions using H2O and alcohols

A

name of mechanism: nucleophilic addition elimination

53
Q

give the general mechanism for acylation reactions using ammonia and 1º amines

A
54
Q

explain what observations during melting point determination would tell you that a compound is not pure

A
  • melts over a range of temperatures
  • melts below the true melting point
55
Q

explain why a pure sample of a compound may appear to melt at a different temperature than the true melting point

A

sample heats up at different rate than thermometer does

the temperarature shown on the thermometer will therefore not be the same as in the sample

56
Q

explain the reasons why impurities can occur in products

A
  • presence of other compounds due to competing reactions
  • contamination with reactants due to incomplete reactions
57
Q

explain how you would carry out recrystallisation

A
  • dissolve impure crystals in small volume of hot solvent (to ensure as much solute is obtained as possible)
  • filter hot solution via gravity filtration using a hot funnel (to ensure crystals dont stick) and fluted filter paper
  • this removes insoluble impurities
  • allow solution to cool and crystallise (lower temp = lower solubility = crystallisation occurs)
  • filter off crystals using suction filtration (as its faster than gravity filtration and provides a drier solid)
  • dry pure crystals between two filter papers
58
Q

explain why nucleophilic addition reactions with acidified can produce a racemic mixture of enantiomers

A

c=o bond is planar , equal probability of nucleophile attacking carbonyl group from above or below the plane

enantiomers only form when the aldehyde or ketone is asymmetrical (different R groups either side)

59
Q

define addition reaction

A

Reaction where a molecule joins to an unsaturated molecule to produce a saturated molecule.

AQA chemistry A level notes Diana Tut (15 decks)

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