The carbonyl group (A2) Flashcards
Define functional group [1]
A group of atoms or an atom responsible for the characteristic reactions of a molecule
Describe the features of the carbonyl group [1]
- -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)
Describe the solubility of aldehydes/ketones in water
- 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
Place these molecules in order of ascending BP (assuming their Mr is similar): aldehydes, ketones, esters, carboxylic acids, alkanes, amines, alcohols and amides
alkanes (VDW forces only) < aldehyde/ketones/esters (dipole-dipole forces) < alcohols/amines (H bonds) < carboxylic acids/amides (H bonds and dipole-dipole forces)
How do you synthesise Aldehydes? include the chemicals and conditions required [2]
- Incomplete oxidation of primary alcohol Conditions: warm with acidified K2Cr2O7 and then distil off products immediately
- Reduction of carboxylic acids using aqueous NaBH4. Condition: heat under reflux with NaBH4 in aqueous ethanol then acidify using dilute H2SO4
Describe the Tollens reagent test
- 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
Describe the Fehling’s solution test
- 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
Describe the reduction of aldehydes/ketones
- 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
Why is NaBH4 a particularly useful reducing agent?
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
Describe the reaction of aldehydes/ketones with hydrogen cyanide (HCN)
- 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
What is the suffix for the ester functional group?
-anoate
Name this compound
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
Draw the functional group for acyl chlorides and describe the naming rule
(number of carbon atom prefic) plus -anoyl chloride
Draw the functional group of acid anhydrides
Name this compound
Ethanoic propanoic anhydride
name each component as a carboxylic acid in alphabetic order
Draw the functional group for amides and describe the naming rule
(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)
Draw the functional group for carboxylate salts
group 1 metal name plus (prefix from number of C atoms attached to C=O group) -anoate
name the amine functional group as a prefix
amino
draw the functional group for alkyl ammonium salts
the H atoms may be substituted by other alkyl groups
Name this compound
But-3-en-1-ol
This is an unsaturated alcohol
-OH takes priority over -C=C-
ene becomes en
Name this compound
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
Name the pairs homologous series that are functional group isomers of each other
- ketones and aldehyde
- alkenes and cycloalkanes
- alcohols and ethers
- esters and carboxylic acids
What are optical isomers
- They are steroisomers which have non-superimposable mirror images
- contain a chiral carbon centre
Describe the 2 types of optical isomers
- 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
Define racemic mixture
- equimolar mixture of + and - enantiomers
- overall optically inactive (equal rotation both clockwise and anticlockwise so rotation cancels out)
How do you separate the different enantiomers in a racemic mixture?
- react the racemic mixture with a chiral reagent so that the products have different physical properties
- alternatively use chromatography
How can you tell if a mechanism will produce a racemic mixture or a single enantiomer?
- 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
Explain the term “homologous series” [3]
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
Define structural formula
Shows the arrangement of the atoms within a formula but does not show the bonds
name this compound
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
describe the 4 ways carboxylic acids can be synthesised
- 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
Describe the acidity of carboxylic acids and give the equation of the reaction of carboxylic acids with water
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
describe the common reactions and condititons of carboxylic acids
- 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
Give the reaction for the synthesis of acyl chlorides
R-COOH + PCl5 → R-COCl + PCl3O + HCl
give the reaction for the synthesis of acid anhydrides
- acid anhydrides are formed via the dehydration of two carboxylic acids
- P2O5 acts as a dehydrating agent
- a molecule of water is released
what are fatty acids?
long chain carboxylic acids
describe the ways to synthesise esters
- 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
give the common reactions of esters
- 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
Describe the test for carboxylic acids
- react with carbonate e.g NaHCO3
- observation = effervescence (also turns colourless limewater cloudy)
Define acylation
- 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
why does heating under reflux ensure complete oxidation of primary alcohols?
- vapours of intermediate aldehyde are condensed and fall back into the flask where they can be further oxidised by the excess oxidising agent
how do you separate esters from the mixture of products formed in esterification reactions?
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
describe the properties of esters
- 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
explain how soap can clean dirt
- 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
why does ethanoic anhydride smell strongly of vinegar?
- it reacts with water vapour in air to reform ethanoic acid which smells of vinegar
describe the properties of acid anhydrides
- insoluble in water
- similar BP to acyl chlorides and esters with the same number of carbons
how are acid anhydrides synthesised?
dehydration of 2 carboxylic acids (either the same or different) using P2O5
give the common reactions of acid anhydrides
include the conditions/observations
- 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)
- reaction with alcohol makes an ester and carboxylic acid
describe the properties of acyl chlorides
- 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
give the common reactions of acyl chlorides
- 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
- reaction with alcohol makes an ester
what are the advantages of using acid anhydrides instead of acyl chlorides in acylation reactions?
- cheaper to make
- produce safer products ( do not release toxic HCl when in contact with water)
- less corrosive
- less vulnerable to hydrolysis
give the general mechanism for acylation reactions using H2O and alcohols
name of mechanism: nucleophilic addition elimination
give the general mechanism for acylation reactions using ammonia and 1º amines
explain what observations during melting point determination would tell you that a compound is not pure
- melts over a range of temperatures
- melts below the true melting point
explain why a pure sample of a compound may appear to melt at a different temperature than the true melting point
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
explain the reasons why impurities can occur in products
- presence of other compounds due to competing reactions
- contamination with reactants due to incomplete reactions
explain how you would carry out recrystallisation
- 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
explain why nucleophilic addition reactions with acidified can produce a racemic mixture of enantiomers
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)
define addition reaction
Reaction where a molecule joins to an unsaturated molecule to produce a saturated molecule.
