15

optical isomer

spatially active isomer (same structure, different arrangement around chiral carbon)

why optical isomerism occurs

result of chirality in molecules with a single chiral centre

enantiomer

optical isomers of each other

chiral compounds are

non-superimposable mirror images

chiral/aasymetric centre

atom bonded to 4 different groups

racemic mixture/racemate

50/50 mixture of 2 isomers (enantiomers)

polarised light

light whose waves vibrate in a single plane

how is polarised light created

by removing most of the light and leaving waves that are in single plane

optical activity

ability of a single optical isomer to rotate the plane of polarisation of plane-polarised monochromatic light in molecules containing a single chiral centre

rotation

• clockwise for + or D

• anticlockwise for - or L

• of enantiomers in opposite directions but by the same number of degrees

racemic mixture is

optically inactive (no effect on plane of light bc half of the molecules rotate one way, half the other)

primary halogenoalkane with central chiral atom + OH

inversion —> single enantiomer, via Sn2

intermediate

• partial bonds

• opposite signs

• 5 bonds

• negative charge

tertiary halogenoalkane with central chiral atom + OH

OH can attack from above or below planar —> racemic mixture of 2 enantiomers, via Sn1

carbonyl group

3-oxobutanoic acid

C=O bond is

polar

aldehydes & ketones VS alkanes VS alcohols bp

1. alcohols

2. aldehydes & ketones, bc pd-pd forces

3. alkanes

aldehydes and ketones form hydrogen bonds

only with water → dissolve in water, but the longer the C chain, the less soluble

test of carbonyl group

2,4-dinitrophenylhydrazine (2,4 - DNPH) → dinitrophenylhydazone

bright orange → yellow

H₂O lost in the process

measuring melting/boiling t helps identifying specific compounds

oxidations & reductions

for 1^y alcohol and aldehyde oxidation

potassium dichromate + heat (distil VS reflux)

for aldehyde oxidation

Tollen’s reagent + heat →silver mirror

Fehling’s solution + heat → blue solution to brick red ppt

for aldehyde and ketone reduction

NaBH₄ (aq)

in oxidation __ produced

1 H₂O molecule

for reduction __ needed

2 [H]

addition of HCN

• nucleophilic addition

• KCN followed by dilute acid

• HCN gas is toxic

• hydroxynitrile formes

• racemic mixture formed

iodine in the presence of alkali

identifies: presence of methyl group next to a carbonyl group

conditions:

• I₂

• NaOH

• gentle heat

naming COOH

carbon in COOH is 1

COOH physical properties governed by

their ability to form H-bonds

COOH bp compared to alcohols

higher:

• hydrogen bonding (H bonds to O that’s = with C)

• 2 molecules form a dimer → double the size, so stronger LF

COOH solubility in water

• dont dimerise

• H-bonds btn COOH and H₂O molecules

• up to 4 C mixes well

1. making COOH

by oxidising 1^y alcohols or aldehydes

• potassium/sodium dichromate

• dilute H₂SO₄

• orange → green

2. making COOH

by hydrolysing a nitrile

• CN^- - a nucleophile

• heat under reflux

• in acidic or basic conditions

• form NH₄ ⁺ or NH₃

• in alkali, ion can be added to dilute acid, which makes COOH

reduction of COOH

• using lithium tetrahydridoaluminate (III) (reacts rapidly, so can’t stop at the aldehyde)

• in dry ether, room t

• RCOOH + 4 [H] → RCH2OH + H₂O (O becomes OH, OH becomes H)

COOH + bases

1. metals → salt + H₂

2. metal OH → salt + H₂O, t increases

3. CO₃ and HCO₃ → salt + H₂O + CO₂, fizzing

salt: (COO^- M⁺)

COOH + PCl₅

COOH + PCl₅ → COCl + POCl₃ + HCl

• steamy acidic fumes of HCl

• liquid mixture of acyl chloride COCl and POCl₃

COOH + alcohols

acidity of COOH

• bc of H in COOH

• when aq, H is transferred to H₂O → H₃O⁺

• COOH (aq) ⇌ COO^- (aq) + H⁺

• reversible so a weak acid; pH 2-3

COOH + NH₃

COOH + NH₃ → NH₄ ⁺

COOH + amines

COOH + RNH₂ → COO^- + RNH₃ ⁺

naming esters

1. alcohol → yl

2. carboxylic acid → oate

draw opposite

formation of esters

• carboxylic acid heated with alcohol

• acid catalyst (conc H₂SO₄)

• slow

• reversible

• water formed

1. hydrolysis of esters

acid catalyst

• ester + H₂O → ^H+ acid + alcohol

2. hydrolysis of esters

base catalyst

• ester + NaOH → COONa (_oate) + alcohol

acyl chloride

• also known as acid chloride

• replacing OH in COOH with Cl

• deriviative of COOH

• more reactive than COOH

• oyl sufix

acyl chloride + water

• Cl replaced with OH

• HCl forms

acyl chloride + alcohol

• Cl replaced with alcohol BUT H

• HCl forms

acyl chloride + conc NH₃

• Cl replaced with NH₂

• HCl forms

acyl chloride + amines

• Cl replaced with amine BUT H

• HCl forms

formation of polymers via condensation

• polyamides: take H from amine, OH from COOH

• polyesters: take H from alcohol, OH from COOH

• H₂O forms

e.g terylene

decoding ¹H/proton NMR spectrum

• no. of peaks/signals: no. of environments

• area under signals: no. of H atoms in each environment → ratio

• no. of splits: how many adjacent (s, d, t, q) + 1

chemical shift for ¹H and ¹³C

the closer the H/C to electronegative element and/or double bond the greater the shift

decoding ¹³C NMR spectrum

• no. of peaks/signals: no. of environments

• no splitting

chromatogrophy

separates components of a mixture using a mobile phase and a stationary phase

the larger the retention time

the greater the interaction of the component with the stationary phase

HPLC and GC

• high-performance liquid chromatography

• gas chromatography

• types of column chromatography that separate substances because of different retention times in the column

• may be used in conjunction with mass spectrometry, in applications such as forensics or drug testing in sport