Topic 17 : Organic Chemistry II 2️⃣ Flashcards
State what is meant by the term chiral
A chiral carbon has 4 different groups attached to it
Forms isomers which are non-superimposable
Give the steps in drawing optical isomers
- Identify the chiral centre
- Draw one enantiomer in a tetrahedral shape
- Draw its mirror image
A substance shows optical activity if…
It rotates the plane of polarisation of plane polarised light
What does a polarimeter measure ?
The amount of optical activity
How does a polarimeter work
A monochromatic light source passes through a polarising filter which converts unpolarised light into plane-polarised light
The plane-polarised light then passes through a sample tube containing some of the substance in solution
If the substance is optically active, the plane of polarisation is rotated
Describe the properties of enantiomers
Identical physical properties with 1 exception :
They both rotate plane-polarised light by equal angles but in opposite directions
Identical chemical properties with 1 exception:
The way in which they react with the enantiomers of other substances may be different for each enantiomer
What is a racemic mixture
Equimolar mixture of 2 enantiomers that has no optical activity as rotations cancel each other out
What are the features of an SN2 reaction mechanism?
- the groups around the central atom get inverted, therefore optical activity of product is different from that of reactant
- 1 step reaction
- we have a transition state
- nucleophile attacks from opposite side to the leaving group
What are the features of an SN1 reaction mechanism
- 2 step reaction
- produces a carbocation with a planar shape so therefore there is equal chance of nucleophile attacking from either side in 2nd step of reaction
- produces 2 products, both of which are enantiomers in a racemic mixture
- optical activity is different from reactant
What is the carbonyl group
A carbon atoms joined by a double bond to an oxygen atom (C=O)
What are the only 2 types of compounds classified as carbonyls ?
Aldehydes and ketones
Describe and explain the bonding in the carbonyl group (C=O)
Composed of a combination of the sigma and pi bond like a C=C bond
C=O bond, however, is polar due to the differing electronegativities of carbon & oxygen - causes a shift in electron density towards the oxygen
Why are carbonyls prone to nucleophilic attack ?
Shift in electrondensity towards the oxygen in the C=O bond causes the carbon to be slightly electron deficient and therefore prone to nucelophilic attack
Describe and explain the boiling temperatures of carbonyl compounds
Contain the polar C=O group so have permanent dipole-dipole attractions
No hydrogen bonding
Boiling temps increase with increasing molecular mass as the extent of London forces increases
Describe and explain the solubility in water of carbonyl compounds
The smaller aldehydes and ketones are soluble in water as they can form hydrogen bonds with water molecules
The solubility decreases with increasing chain length as the hydrocarbon portion of the molecule becomes more significant
How are aldehydes and ketones converted to alcohols ?
Reduction using the reagent lithium tetrahydridoaluminate (LiAlH4)
Reaction is carried out with both the carbonyl compound and the reagent dissolved in dry ether
The reducing agent can be represent by a [H] in equations
What are the 4 different reagents that can be used to oxidise aldehydes to carboxylic acids
Acidified potassium dichromate (VI)
Fehling’s solution
Benedict’s solution
Tollens’ reagent
What colour change can be seen for the oxidation of aldehydes with Fehling’s or Benedict’s solution
Deep blue -> red precipitate
What colour change can be seen for the oxidation of aldehydes to carboxylic acids using Tollens’ reagent
Colourless solution —> silver mirror
Describe the reaction of a carbonyl with iodine
Redox reaction
Carbonyl compound is added to an alkaline solution iodine
Mixture is warmed and then cooled
Pale yellow precipitate forms = positive result for ethanal or a ketone
Describe the reaction of carbonyls with hydrogen cyanide
- carried out in an aqueous alkaline solution containing KCN
- reagent is HCl
- nucleophilic addition
- products are referred to as hydroxynitriles
Describe and explain the nucleophilic addition mechanism of the reaction of carbonyls and HCl
Step 1 : involves the nucleophilic attack by a :CN- on the carbon atom of the carbonyl compound
Step 2: involves the reaction between the intermediate and the H-CN molecule
Results in a product where the functional groups -OH and -CN are joined to a carbon atom forming hydroxynitriles
Describe the reaction of Brady’s reagent (2,4 dinitrophenylhydrazine) with carbonyl compounds
Reacts with most carbonyl compounds to form a bright orange precipitate
Describe and explain the boiling temperatures of carboxylic acids
Strong intermolecular forces (hydrogen bonding)
As carbon chain length increases so does boiling point due to increased London forces
Pure carboxylic acids can form hydrogen bonded dimers
Describe and explain the solubility of carboxylic acids in water
Shorter chain carboxylic acids are soluble in water as they can form hydrogen bonds with water molecules
Solubility decreases with increasing chain length as the hydrocarbon portion of the molecule becomes larger
What are the 2 methods of preparing carboxylic acids
- preparation by oxidation
- preparation by hydrolysis (acidic and alkaline)
Describe how carboxylic acids are prepared by oxidation
- uses either a primary alcohol or aldehyde as starting material
- oxidising agent of acidified potassium dichromate (VI)
- method: heat mixture under reflux
- when oxidation is complete, reaction mixture is fractionally distilled to obtain pure sample of carboxylic acid
Describe how carboxylic acids are prepared by hydrolysis (acidic and alkaline)
- nitriles (organic compounds containing CN group) are the starting material
- can be hydrolysed by heating under reflux with either a dilute acid or aqueous alkali
- CN bond breaks and the nitrogen atom becomes either ammonia or the ammonium ion
Give the equation for acidic hydrolysis to produce a carboxylic acid
CH3CH2CN + H+ + 2H2O ——-> CH3CH2COOH + NH4+
Give the equation for alkaline hydrolysis to produce a carboxylic acid
CH3CH2CH2CN + OH- + H2O —-> CH3CH2CH2COO- + NH3
Product: butanoate ion
Can be converted to butanoic acid easily by adding dilute acid
CH3CH2CH2COO- + H+ —-> CH3CH2CH2COOH
What are the 4 reactions of carboxylic acids
- reduction
- neutralisation
- esterification
- halogenation
Describe the reduction reaction of a carboxylic acid
- reducing agent of lithium tetrahydrioaluminate
- solvent of dry ether
- reduced to aldehydes and primary alcohols
Describe the neutralisation reaction of a carboxylic acid
- although a weak acid, they can be neutralised by mixing with aqueous alkali such as NaOH
- products are carboxylate salts (general formula RCOONa) and water
Describe the halogenation reaction of a carboxylic acid
- reagent of phosphorus(V) chloride
- anhydrous conditions
- OH group is replaced by halogen atom so functional group becomes -COCl for example
- products known as acyl chlorides
- POCl3 is also produced as well as a hydrogen halide such as HCl
Questions on halogenation of carboxylic acids:
- why must reaction be under anhydrous conditions ?
- what must happen to product POCl3 and why?
- why do misty fumes appear?
- both the reagent and the acyl chloride product react with water
- must be separated by fractional distillation as it is a liquid that mixes with acyl chloride
- hydrogen halide gas produced escapes
Describe the esterification reaction of a carboxylic acid
- carboxylic acid is mixed with an alcohol and a small amount of acid catalyst (often conc H2SO4)
- reaction is slow and reversible
- ester and water are produced
What is the general formula of acyl chlorides
RCOCl
Why are acyl chlorides prone to nucleophilic attack?
The carbon atom in RCOCl is joined to 2 electronegative atoms and so is electron deficient (δ+)
Describe the reaction of acyl chlorides with water
Acyl chlorides react vigorously with cold water, forming a carboxylic acid and releasing HCl gas, which appears as misty fumes
Describe the reaction of acyl chlorides with alcohols
Acyl chlorides react readily with ethanol to form an ester and HCl gas
Describe the reaction of acyl chlorides with concentrated ammonia solution
RCOCl + NH3 —-> RCONH2 + HCl
- forms an amide and HCl
further reaction as NH3 is a base and HCl is an acidic gas…
NH3 + HCl —-> NH4Cl
- ammonium chloride is formed
Describe the reaction of acyl chlorides with amines
- acyl chlorides react with primary amines (RNH2) to produce a substituted amide (often called an N-substituted amide)
- HCl is also produced
CH3COCl + CH3NH2 —-> CH3CONHCH3 + HCl - acyl chlorides also react with secondary amines (R2NH)
- product formed will contain 2 substituted alkyl groups
CH3COCl + (CH3)2NH —-> CH3CON(CH3)2 + HCl
How do you name esters
Example CH3COOCH2CH2CH3
- 1st word from alkyl group attached to O
- 2nd word from alkyl group attached to C
Therefore…
Propyl ethanoate
What are the physical properties of esters
Generally colourless liquids
Relatively low melting and boiling temps
Insoluble in water
What is the general formula of an ester
RCOOR’
What is the general formula of nitriles
RCN
What is the general formula of hydroxynitriles
CN
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R— C— OH
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R
