17: Chirality and Carbonyls Flashcards
Define optical isomers
Isomers of which one is the non-superimposable mirror image of the other
What is chirality caused by?
A carbon with 4 different groups attached to it
Called a chiral centre
What are optical isomers known as?
Enantiomers
Why are chiral molecules known as optical isomers?
The isomer can rotate the plane of polarisation of plane-polarised monochromatic light
Define a racemic mixture
A solution containing equal amounts of 2 isomers
Does not rotate plane of plane-polarised monochromatic light
Compare and contrast enantiomers
Identical physical and chemical properties
React differently with plane-polarised light
What is the structure of an aldehyde functional group?
Carbon bonded to a H, double bond O, and a R group
Found on the end of a chain
What is the structure of a ketone functional group?
Carbon with a double O bond, and two R groups
Found in the middle of a carbon chain
What is significant about a carbon oxygen double bond?
Oxygen is more electronegative, causes a polar bond
Why can aldehydes and ketones not hydrogen bond with each other?
No hydrogen atoms bonded to the oxygen atoms
Are ketones and aldehydes soluble in water?
Small aldehydes and ketones are soluble in water
Solubility decreases as carbon chain increases
Why are only small carbonyls soluble in water?
They can hydrogen bond with water molecules
Hydrocarbon tails are hydrophobic
Solubility breaks hydrogen bonding in water, and if they are not replaced by strong inter molecular bonds then it will be insoluble
Hydrocarbon chains only have id-id so the longer, the less soluble
What is the main form of reactions carbonyls undergo and why?
Nucleophillic addition
Condensation
Due to 𝛿+ carbon atom attracting nucleophiles
What is the test for a carbonyl group?
Adding 2,4-DNP
Forms yellow crystals (2,4-dinitrophenylhydrazone crystals)
Condensation reaction
What are the 3 oxidation tests for aldehydes (not ketones)?
Tollens reagent - ammonial silver nitrate
Fehlings solution - copper 2+ ions
Acidified dichromate - H+/Cr2O7 2-
Why are oxidation tests used to test for aldehydes not ketones?
Aldehydes can be further oxides ex to carboxylic acids
Ketones cannot be futher oxidised
How is tollens reagent formed?
Ag+ + 2NH3 -> Ag[NH3]2 +
What are the half equations for the tollens reagent in presence of an aldehyde?
R-CHO + H2O -> R-COOH + 2e + 2H+
Ag+ + e -> Ag
Combined:
R-CHO + H2O + 2Ag+ -> R-COOH + 2Ag + 2H+
What is observed in the Tollens reagent test in the presence of an aldehyde?
A silver mirror will form on the inside of the test tube
What is in fehlings solution?
Solution of Cu2+ ions
What is reduced in the Fehlings test?
Cu2+ -> Cu+
What are the half equations for the Fehlings test?
R-CHO + H2O -> R-COOH + 2e + 2H+
2Cu2+ + H2O + 2e -> Cu2O + 2H+
Combined:
R-CHO + 2H2O + 2Cu -> R-COOH + Cu2O+ 4H+
What is the colour change if acidified potassium dichromate is in solution with an aldehyde?
Orange to green
What is the change in oxidation state when potassium dichromate is oxidised?
Cr6+ -> Cr3+
What is the half equation for potassium dichromate reacting with an aldehyde?
Cr2O7 2- + 14H+ + 6e -> 2Cr 3+ + 7H2O
R-CHO + H2O -> R-COOH + 2e + 2H+
Combined:
3R-CHO + Cr2O7 2- + 8H+ -> 3R-COOH + 2H2O + 2Cr 3+
How does the increase of chain length effect the oxidation of an aldehyde?
Slower they react
The aldehydes have less reducing power
Why do ketones not give a positive result in reducing tests?
They cannot be oxidised easily
What is the iodoform (triiodomethane) test?
Tests for the methyl carbonyl and methyl OH groups
What are the reagents in the iodoform test?
Iodine and sodium hydroxide
What is the sign for a positive iodoform test?
CHI3 - Pale yellow precipitate
Antiseptic smell
What is the equation for the iodoform test?
R-CHO + 3I2 + 4NaOH -> CHI3 + HCOO-Na+ + 3NaI + 3H2O
What is produced when cyanide and hydrogen cyanide is added to a carbonyl?
Carbonyl -> organic compound with a cyandide and alcohol
What are the steps in the nucleophillic addition of cyanide to a ketone?
Lone pair on CN- attacks 𝛿+ Carbon
One of double bond breaks and both electrons goes to the Oxygen
Lone pair on oxygen forms bond with Hydrogen from HCN
CN- ion and organic product is left
What can the product of the addition of cyanide be converted into?
Anime - LiAlH4 and dry ether
Carboxylic acid - H2O/H+
What is LiAlH4 and what does it do?
Lithium aluminium hydride
Reduces C=O bond to OH
Reduces CN bond to NH2
What is the first step in the addition of HCN to a carbonyl?
Attack by CN- of 𝛿+ on C=O
What are the similarities and differences of enantiomers?
Same molecular formula
Same structural formula
Atoms in different position in 3D space
Why is SN2 called this?
SN - Nucleophillic substitution
2 - 2 species involved in the first step
What happens to optical isomers in an SN2 reaction?
The molecule will retain its chirality
What type of halogenoalkanes undergo SN2?
Primary
What type of halogenoalkanes undergo SN1?
Tertiary
What is the effect of SN1 reactions on optical isomers?
Racemic mix is formed
Intermediate is trigonal planar so the nucleophile can be added on either side
What is the effect of the SN2 mechanism on optical isomers?
No effect on optical isomers
Single enantiomer reactant produces single enantiomer product
What is the difference between addition and condensation polymerisation?
Addition - All monomer atoms present in polymer
Condensation - Water lost
What are the typical monomers of condensation polymerisation?
Diol + dicarboxylic acids
How are carboxylic acids arranged without water?
Dimers - hydrogen bond with another carboxylic acid
How is a carboxylic acid produced from a nitrile?
Reflux with dilute acid in aqueous environments
What is the general formula for the production of carboxylic acids from nitriles?
R-CN + H2O + H+ -> R-COOH + NH4+
How do carboxylic acid reaction with sodium hydroxide?
Acid + Alkali -> Salt + Water
R-COOH + NaOH -> R-COO- Na+ + H2O
How do carboxylic acids react with alcohols?
Carboxylic acid + Alcohol -> Ester + Water
Presence of acid catalyst needed - conc sulphuric acid
What is the reaction type of the formation of a ester?
Esterification - Adddition elimination
What are the conditions for esterification?
Heated with acid catalyst (sulphuric acid)
Slow and reversible reaction
How are esters named?
Take name of alcohol and replace “anol” with “yl”
Second word is carboxylic acid, replace “oic” with “anoate”
What occurs in the reaction between LiAlH4 and carboxylic acids?
Carboxylic acid + H- -> Alcohol (OH)
Organic solvent - dry ether
How does LiAlH4 work?
H- ion acts as a nucleophile
Will reduce water so organic solvent must be used
How are esters made into alcohols and carboxylic acids?
Hydrolysed
How are acyl chlorides formed?
R-COOH + PCl5 -> R-OCl + POCl3 + HCl
Releases steamy fumes of HCl
How can the presence of HCl fumes be confirmed?
Will produce white smoke in presence of ammonia
Forms ammonium chloride
What is the functional group of acyl chlorides?
R-O-Cl
How do acyl chlorides typically react?
Nucelophilic substitution
What is produced when acyl chlorides react with water?
Carboxlyic acid + HCl
What is the defining property of acyl chlorides?
High reactivity
Caused by large difference in electronegativity between Cl and C - draws electrons away creating a larger positive charge
What is a common alternative to acyl chlorides?
Ethanoic anhydride
What is the problem when acyl chlorides being used as a reactant?
Toxic fumes produced
Fume cupboard should be used
Which bonds are broken and made in esterification?
Broken: C-O, O-H
Made: C-O, O-H
How do esters react with NaOH?
R-O-R’ + NaOH -> RCOONa + R’-OH
What is produced when acyl chlorides react with alcohols (OH)?
Ester + HCl
What is produced when acyl chlorides react with amines?
N-substituted amide + HCl
What is produced when acyl chlorides react with concentrated ammonia?
Amide + HCl
What are the reactions like which involve acyl chlorides?
Vigorous reaction at room temp
