IMAT Chemistry · Free notes

Organic Chemistry: Structures, Names, Isomers and Reactions

Organic Chemistry has 31 past questions since 2011, and the ministry papers from 2023 on have asked 10 of them. The ministry questions are short recognition tasks: which family a compound belongs to, which group is in a drawn drug molecule, or what shape benzene has, and two of them were about amino acids and sugars. The older Cambridge papers were harder, with isomer counting and multi-step reaction schemes. The difficulty sits in reading a name or a formula fast and accurately, so practise turning names into structures until it is automatic.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

IMAT home All formulas on one page

Subtopic notes

Formula & revision sheet

7 formulas · 10 reference tables · 26 gotchas across all subtopics — the exam-eve cheat-sheet

How Carbon Bonds, and How We Write Organic Molecules

Formulas (1)

Molecular, condensed and skeletal formulas, and counting the hydrogens · Hidden hydrogens on a skeletal carbon $n_{\mathrm{H}} = 4 - (\text{number of bonds drawn to that carbon})$

Reference tables (1)

Carbon's four bonds and its hybridisation (sp³, sp², sp)3 rows

Hybridisation	Atoms joined to the carbon	Shape and bond angle	Example
$\mathrm{sp^3}$	4 (four single bonds: 4 σ)	Tetrahedral, about 109.5°	Both carbons of ethane $\mathrm{CH_3CH_3}$ ; the carbon of $\mathrm{CH_4}$
$\mathrm{sp^2}$	3 (one double bond: 3 σ and 1 π)	Trigonal planar, 120°	Both carbons of ethene $\mathrm{CH_2{=}CH_2}$ ; the C of a C=O group; every carbon of benzene
$\mathrm{sp}$	2 (a triple bond, or two double bonds: 2 σ and 2 π)	Linear, 180°	Both carbons of ethyne $\mathrm{HC{\equiv}CH}$ ; the carbon of $\mathrm{CO_2}$

Count atoms, not bonds: a carbon in a C=C still has four bonds, but it is joined to only three atoms, so it is sp².

Watch out for (3)

A double bond is one σ and one π, not two π bonds
→ Carbon's four bonds and its hybridisation (sp³, sp², sp)
Hybridisation follows the number of atoms joined, not the number of bonds
→ Carbon's four bonds and its hybridisation (sp³, sp², sp)
The hydrogens you cannot see in a skeletal formula are still there
→ Molecular, condensed and skeletal formulas, and counting the hydrogens

Hydrocarbons: Families, Benzene, Names and Unsaturation

Formulas (2)

IUPAC names for alkanes and alkenes · Order of an IUPAC name $\text{locants-branches} + \text{stem} + \text{ending}$
Degrees of unsaturation: rings and multiple bonds from a formula · Degree of unsaturation $\text{DoU} = \frac{2C + 2 + N - H - X}{2}$

Reference tables (2)

Hydrocarbon families and their general formulas5 rows

Family	General formula	Key feature	Example	Name ending
Alkane	$\mathrm{C_nH_{2n+2}}$	Only single bonds, open chain	Propane $\mathrm{C_3H_8}$	-ane
Cycloalkane	$\mathrm{C_nH_{2n}}$	Single bonds in one ring	Cyclohexane $\mathrm{C_6H_{12}}$	cyclo-, then -ane
Alkene	$\mathrm{C_nH_{2n}}$	One C=C double bond	Propene $\mathrm{C_3H_6}$	-ene
Alkyne	$\mathrm{C_nH_{2n-2}}$	One C≡C triple bond	Propyne $\mathrm{C_3H_4}$	-yne
Arene (aromatic)	$\mathrm{C_nH_{2n-6}}$ for one benzene ring	A benzene ring	Methylbenzene $\mathrm{C_7H_8}$	-benzene

Alkenes and cycloalkanes share

\mathrm{C_nH_{2n}}

; a formula alone cannot tell them apart.

Benzene and aromatic compounds5 rows

Property	Benzene	Cyclohexane, for contrast
Formula	$\mathrm{C_6H_6}$	$\mathrm{C_6H_{12}}$
Hybridisation of every carbon	$\mathrm{sp^2}$	$\mathrm{sp^3}$
Shape	Flat hexagon, 120° angles	Puckered ring (chair or boat shape)
C-C bonds	Six equal bonds, delocalised π electrons	Six single bonds
Typical reaction	Substitution (keeps the ring)	Burns; substitution with halogens in UV light

Watch out for (4)

Ethene is an alkene, not an alkyne
→ Hydrocarbon families and their general formulas
Benzene is flat; the chair and boat shapes belong to cyclohexane
→ Benzene and aromatic compounds
The longest chain is not always the one written in a straight line
→ IUPAC names for alkanes and alkenes
A ring removes two hydrogens just like a double bond
→ Degrees of unsaturation: rings and multiple bonds from a formula

Functional Groups: Recognise, Name and Find Them in Biomolecules

Formulas (1)

Naming compounds that contain a functional group · Name with a functional group $\text{branches} + \text{stem} + \text{locant} + \text{group ending}$

Reference tables (2)

The main functional groups and their families11 rows

Family	Group	Name ending	Example
Haloalkane	C-X, where X is F, Cl, Br or I	prefix chloro-, bromo-	Chloroethane $\mathrm{CH_3CH_2Cl}$
Alcohol	$\mathrm{-OH}$ on a saturated carbon	-ol	Ethanol $\mathrm{CH_3CH_2OH}$
Ether	C-O-C, no C=O	alkoxy- prefix, or 'dimethyl ether' style	Methoxymethane $\mathrm{CH_3OCH_3}$
Aldehyde	$\mathrm{-CHO}$ : C=O at the end of a chain	-al	Ethanal $\mathrm{CH_3CHO}$
Ketone	C=O between two carbons	-one	Propanone $\mathrm{CH_3COCH_3}$
Carboxylic acid	$\mathrm{-COOH}$	-oic acid	Ethanoic acid $\mathrm{CH_3COOH}$
Ester	$\mathrm{-COO-}$ between two carbons	alkyl ...-oate	Methyl ethanoate $\mathrm{CH_3COOCH_3}$
Amine	N bonded to C and H, no C=O next to it	-amine	Ethylamine $\mathrm{CH_3CH_2NH_2}$
Amide	$\mathrm{-CONH_2}$ : C=O bonded to N	-amide	Ethanamide $\mathrm{CH_3CONH_2}$
Nitrile	$\mathrm{-C{\equiv}N}$	-nitrile	Ethanenitrile $\mathrm{CH_3CN}$
Thiol	$\mathrm{-SH}$	-thiol	Ethanethiol $\mathrm{CH_3CH_2SH}$

An OH on a benzene ring is a phenol, a separate family that is more acidic than an alcohol.

Functional groups in biological molecules5 rows

Molecule	Made from	Linking group	Key fact
Protein (polypeptide)	Amino acids	Peptide bond (amide)	Every amino acid has NH₂ and COOH; cysteine also has a thiol SH
Starch	Glucose	Glycosidic link (C-O-C)	Energy store in plants
Glycogen	Glucose	Glycosidic link (C-O-C)	Energy store in animals (liver and muscle), not in plants
Disaccharide (sucrose, lactose)	Two monosaccharides	One glycosidic link	Sucrose is glucose + fructose
Triglyceride (fat, oil)	Glycerol and three fatty acids	Three ester groups	Insoluble in water; soluble in non-polar solvents

Watch out for (6)

Amine or amide: look for the C=O next to the nitrogen
→ The main functional groups and their families
Ethers have no C=O at all
→ The main functional groups and their families
The OH of a COOH group is not an alcohol
→ Finding every functional group in a larger molecule
Esters are named alcohol part first
→ Naming compounds that contain a functional group
Glycogen is a homopolysaccharide stored by animals, not plants
→ Functional groups in biological molecules
Amino acids are water-soluble ions, not oily molecules
→ Functional groups in biological molecules

Isomerism: Structural Isomers, Cis/Trans and Chirality

Formulas (1)

Chirality: chiral carbons and optical isomers · Maximum number of stereoisomers $N_{\max} = 2^{n}$

Reference tables (1)

Cis/trans (geometric) isomerism around a C=C5 rows

Compound	Groups on the two C=C carbons	Cis/trans isomers?
But-2-ene $\mathrm{CH_3CH{=}CHCH_3}$	H and $\mathrm{CH_3}$ ; H and $\mathrm{CH_3}$	Yes
But-1-ene $\mathrm{CH_2{=}CHCH_2CH_3}$	H and H; H and $\mathrm{C_2H_5}$	No: carbon 1 has two H
2-methylbut-2-ene $\mathrm{(CH_3)_2C{=}CHCH_3}$	$\mathrm{CH_3}$ and $\mathrm{CH_3}$ ; H and $\mathrm{CH_3}$	No: carbon 2 has two $\mathrm{CH_3}$
1,2-dichloroethene $\mathrm{CHCl{=}CHCl}$	H and Cl; H and Cl	Yes
1,1-dichloroethene $\mathrm{CCl_2{=}CH_2}$	Cl and Cl; H and H	No

Watch out for (5)

Isomers need not have similar physical properties
→ Structural isomers: chain, position and functional group
A formula with one oxygen and one degree of unsaturation is not only a carbonyl compound
→ Structural isomers: chain, position and functional group
A C=C is needed for cis/trans, but it is not enough
→ Cis/trans (geometric) isomerism around a C=C
Enantiomers have the same physical properties
→ Chirality: chiral carbons and optical isomers
A carbon in a double bond cannot be a chiral centre
→ Chirality: chiral carbons and optical isomers

Organic Reactions: Types, Oxidation, Esters and Combustion

Formulas (2)

Esterification, hydrolysis and amide formation · Esterification $\mathrm{RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O}$
Combustion of organic compounds · Complete combustion of a fuel containing C, H and O $\mathrm{C_xH_yO_z} + \left(x + \frac{y}{4} - \frac{z}{2}\right)\mathrm{O_2} \rightarrow x\,\mathrm{CO_2} + \frac{y}{2}\,\mathrm{H_2O}$

Reference tables (2)

The main types of organic reaction6 rows

Type	What happens	Example	How to spot it
Substitution	One atom or group replaces another	$\mathrm{CH_3CH_2Br + OH^- \rightarrow CH_3CH_2OH + Br^-}$	Two reactants, two products; no change in multiple bonds
Addition	Two molecules join across a C=C or C≡C	$\mathrm{CH_2{=}CH_2 + Br_2 \rightarrow CH_2BrCH_2Br}$	One product; a multiple bond is used up
Elimination	A small molecule leaves neighbouring carbons, making a C=C	$\mathrm{CH_3CH_2OH \rightarrow CH_2{=}CH_2 + H_2O}$	A new C=C appears
Condensation	Two molecules join and release a small molecule, usually water	Acid + alcohol $\rightarrow$ ester + water	Water among the products
Hydrolysis	Water splits a molecule; the reverse of condensation	Ester + water $\rightarrow$ acid + alcohol	Water among the reactants
Oxidation	Carbon gains O or loses H	Ethanol $\rightarrow$ ethanal $\rightarrow$ ethanoic acid	More O or fewer H in the product

Oxidation of primary, secondary and tertiary alcohols3 rows

Alcohol class	Example	Product of oxidation	Dichromate colour
Primary	Propan-1-ol $\mathrm{CH_3CH_2CH_2OH}$	Propanal, then propanoic acid	Orange to green
Secondary	Propan-2-ol $\mathrm{CH_3CH(OH)CH_3}$	Propanone (a ketone) only	Orange to green
Tertiary	2-methylpropan-2-ol $\mathrm{(CH_3)_3COH}$	No reaction	Stays orange

Watch out for (5)

Adding bromine to an alkene is also an oxidation of carbon
→ The main types of organic reaction
Condensation joins molecules; elimination splits one
→ The main types of organic reaction
A primary alcohol never gives a ketone
→ Oxidation of primary, secondary and tertiary alcohols
Base hydrolysis gives a salt, not the free acid
→ Esterification, hydrolysis and amide formation
Subtract the oxygen the fuel already contains
→ Combustion of organic compounds

Physical Properties: Boiling Points and Solubility

Reference tables (2)

Boiling points of organic compounds: size, branching and hydrogen bonds5 rows

Factor	Effect on boiling point	Example pair (approximate boiling points)
Longer chain	Higher	Butane −1 °C < pentane 36 °C
More branching	Lower	2,2-dimethylpropane 10 °C < pentane 36 °C
Hydrogen bonding (O-H)	Much higher	Dimethyl ether −24 °C < ethanol 78 °C (both $M = 46$ )
Polar C=O	Higher than an alkane	Butane −1 °C < propanone 56 °C (both $M = 58$ )
Two hydrogen bonds per pair (acid)	Highest for its size	Propan-1-ol 97 °C < ethanoic acid 118 °C (both $M = 60$ )

Solubility of organic compounds in water and in non-polar solvents5 rows

Compound type	In water	In hexane	Reason
Ethanol, ethanoic acid	Mixes completely	Soluble	Small, hydrogen bonds with water
Propanone	Mixes completely	Soluble	C=O accepts hydrogen bonds from water
Hexan-1-ol	Slightly soluble	Soluble	Long non-polar tail outweighs the OH
Hexane, chloroethane	Insoluble	Soluble	No hydrogen bonding with water
Sodium ethanoate, glycine	Soluble	Insoluble	Ionic

Watch out for (3)

Boiling breaks forces between molecules, not covalent bonds
→ Boiling points of organic compounds: size, branching and hydrogen bonds
Branching lowers the boiling point even with the same formula
→ Boiling points of organic compounds: size, branching and hydrogen bonds
A molecule does not need its own O-H to dissolve in water
→ Solubility of organic compounds in water and in non-polar solvents

PYQ weightage by concept

19 concepts · 0 PYQs — where the marks actually sit, so you know what to drill first

How Carbon Bonds, and How We Write Organic Molecules0 PYQs · 0%

Concept	PYQs	Share
Carbon's four bonds and its hybridisation (sp³, sp², sp)foundation	—	—
Molecular, condensed and skeletal formulas, and counting the hydrogensfoundation	—	—

Hydrocarbons: Families, Benzene, Names and Unsaturation0 PYQs · 0%

Concept	PYQs	Share
Hydrocarbon families and their general formulasfoundation	—	—
Benzene and aromatic compoundsfoundation	—	—
IUPAC names for alkanes and alkenesfoundation	—	—
Degrees of unsaturation: rings and multiple bonds from a formulafoundation	—	—

Functional Groups: Recognise, Name and Find Them in Biomolecules0 PYQs · 0%

Concept	PYQs	Share
The main functional groups and their familiesfoundation	—	—
Finding every functional group in a larger moleculefoundation	—	—
Naming compounds that contain a functional groupfoundation	—	—
Functional groups in biological moleculesfoundation	—	—

Isomerism: Structural Isomers, Cis/Trans and Chirality0 PYQs · 0%

Concept	PYQs	Share
Structural isomers: chain, position and functional groupfoundation	—	—
Cis/trans (geometric) isomerism around a C=Cfoundation	—	—
Chirality: chiral carbons and optical isomersfoundation	—	—

Organic Reactions: Types, Oxidation, Esters and Combustion0 PYQs · 0%

Concept	PYQs	Share
The main types of organic reactionfoundation	—	—
Oxidation of primary, secondary and tertiary alcoholsfoundation	—	—
Esterification, hydrolysis and amide formationfoundation	—	—
Combustion of organic compoundsfoundation	—	—

Physical Properties: Boiling Points and Solubility0 PYQs · 0%

Concept	PYQs	Share
Boiling points of organic compounds: size, branching and hydrogen bondsfoundation	—	—
Solubility of organic compounds in water and in non-polar solventsfoundation	—	—