⬛️ Topic 17 - Organics II Flashcards
What are the observations when sodium/potassium dichromate is added to an aldehyde? What gives the colour?
Orange —> green (due to Cr3+ ions)
(Sodium dichromate is an oxidising agent [O] and an aldehyde will oxidise (to a carboxylic acid))
What are the observations when sodium/potassium dichromate is added to ketone?
No observations as sodium dichromate is an oxidising agent and a ketone will not oxidise
What are the observations when sodium/potassium dichromate is added to a primary/secondary alcohol?
Orange —> green
(As primary and secondary alcohols will oxidise)
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?
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)
What are the observations when Benedict’s solution is added to a ketone?
Nothing - a ketone does not oxidise
What are the observations when Tollens is added to an aldehyde? What is Tollens made from?
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)
What are the observations when Tollens is added to a ketone?
Nothing - a ketone will not oxidise
What are the observations when Fehling’s is added to an aldehyde?
Blue —> red precipitate
(Fehling’s is an oxidising agent [O] and an aldehyde will oxidise)
What are the observations when Fehling’s solution is added to a ketone?
Nothing - a ketone will not oxidise
What is the qualitative test for C=O (a carbonyl group)? What is a positive result? How can you identify the specific carbonyl compound?
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)
How do you reduce an aldehyde back to a primary alcohol/ reduce a ketone back to a secondary alcohol?
React with lithium tetrahydridoaluminate (LiAlH4) in dry ether [H]
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?
Reaction with iodine in alkali (iodoform) - a yellow precipitate (CHI3 - iodoform) forms from a colourless solution, with a ‘hospital’/antiseptic smell
How do you make a hydroxynitrile (-OH and -CN group) from a carbonyl compound (compound with a C=O)? What is the mechanism called?
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)
Order, from lowest to highest, the boiling points of aldehydes/ketones, alkanes, carboxylic acids and alcohols. Give reasons.
- 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?)
Order, from lowest to highest, the solubility in water (/polar solvents) of aldehydes/ketones, alkanes, carboxylic acids and alcohols. Give reasons.
- 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)
What is produced when a carboxylic acid is reacted with a base? What do the products all have in common?
A carboxylate salt is formed - always soluble (eg. Lithium ethanoate, Potassium butanoate)
(Neutralisation)
How do you reduce a carboxylic acid? Reagent? What is the product?
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)
How do you halogenate a carboxylic acid? Reagents/conditions? Product? Observations?
- 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
What does the esterification of a carboxylic acid involve? Reagents/conditions? Products?
What does the esterification of an acyl chloride involve?
- 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)
How do you prepare a carboxylic acid by oxidation? Oxidation of what? Oxidising agent? Conditions? Method?
- 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
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?
- 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
What is significant about reactions of acyl chlorides?
- 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
How do you make a carboxylic acid from an acyl chloride?
Reagent: (cold) water
Conditions: room temperature
(Acyl chlorides react with water, so don’t dissolve - aren’t soluble)
How do you make a primary amide from an acyl chloride? What might the by-product be?
Reagent: concentrated ammonia solution
Conditions: room temperature
(NH4Cl may be produced instead of HCl as the NH3 will react with the HCl)
How do you make a secondary/tertiary amide (N-substituted amides) from an acyl chloride?
Reagent: primary amine / secondary amine (respectively)
Conditions: room temperature
What are the two types of hydrolysis of esters?
- 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] - 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+))
Explain the relative boiling point and solubility of esters
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
What type of polymerisation is the formation of polyesters, and why is this different to the other type?
- 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)
Why are diacyl chlorides generally not used in place of dicarboxylic acids when making polyesters?
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
Why can polymers not be used in acidic conditions?
If the polymer is in prolonged contact with an acid, hydrolysis will occur
What is an optical isomer?
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]
What is optical activity? What characteristic do all optical isomers have?
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)
What are the differences and similarities between two optical isomers (enantiomers) (L- and D-)
Similarities:
- have identical chemical + physical properties
Differences:
- effect on plane polarised light (direction of rotation)
- their reactions with other chiral molecules
What is polarimetry, and what effect do the enantiomers have?
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
Can there be a chiral centre on a cyclic structure or a benzene ring?
There can be a chiral carbon in a cyclic structure but NOT a benzene ring
If there are 2 chiral centres in a molecule, how many optical isomers are there?
4 optical isomers (as 4 combinations of +/- forms)
What is a racemic mixture?
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)
What does it mean if there is no rotation of plane polarised light?
There are no optical isomers OR a racemic mixture is formed
What are the two types of nucleophilic substitution mechanisms, and what’s the difference between them?
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
What are two uses of esters?
Perfumes and flavourings
