IMAT Chemistry · Formula sheet

Organic Chemistry formulas

7 formulas, 10 reference tables and 26 common traps for IMAT Chemistry Organic Chemistry, grouped by subtopic.

How Carbon Bonds, and How We Write Organic Molecules

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})

$n_{\mathrm{H}}$ hydrogens on that carbon
bonds drawna double line counts 2, a triple line counts 3

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

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.

Common traps

A double bond is one σ and one π, not two π bonds

The first bond between two atoms is always σ. Only the second and third bonds are π. So a double bond has 1 σ + 1 π and a triple bond 1 σ + 2 π. Options that call a double bond two π bonds are wrong.

Hybridisation follows the number of atoms joined, not the number of bonds

Every carbon makes four bonds, so counting bonds tells you nothing. Count the atoms attached: 4 gives sp³, 3 gives sp², 2 gives sp.

The hydrogens you cannot see in a skeletal formula are still there

A skeletal formula hides the hydrogens on carbon but shows the ones on O and N. Forgetting the hidden ones gives a formula short of hydrogen; forgetting the drawn OH hydrogen loses one more. Count carbon by carbon with

4 - \text{bonds drawn}

Hydrocarbons: Families, Benzene, Names and Unsaturation

Learn this subtopic in the notes

IUPAC names for alkanes and alkenes

Order of an IUPAC name

\text{locants-branches} + \text{stem} + \text{ending}

branchesalphabetical, with their carbon numbers
stemthe number of carbons in the longest chain
ending-ane, -ene (with its number), -yne

Degrees of unsaturation: rings and multiple bonds from a formula

Degree of unsaturation

\text{DoU} = \frac{2C + 2 + N - H - X}{2}

$C, H, N$ numbers of carbon, hydrogen and nitrogen atoms
$X$ number of halogen atoms (F, Cl, Br, I)

Hydrocarbon families and their general formulas

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 compounds

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

Common traps

Ethene is an alkene, not an alkyne

The letters before the ending count the carbons (eth = 2) and the ending gives the bond: -ane single, -ene double, -yne triple. Ethene

\mathrm{CH_2{=}CH_2}

has one double bond, so it is an alkene. A diene has two C=C bonds and needs at least three carbons.

Benzene is flat; the chair and boat shapes belong to cyclohexane

Benzene's carbons are all sp² and the ring is planar. Cyclohexane's carbons are sp³, so its ring folds into chair or boat shapes. Options that mix the two (sp² but chair-shaped, or some sp³ carbons in benzene) are wrong.

The longest chain is not always the one written in a straight line

Condensed and skeletal formulas often bend the main chain through a branch. Check every path from one end carbon to another; a molecule written as a butane with an ethyl branch may really be a pentane or hexane chain.

A ring removes two hydrogens just like a double bond

Cyclohexane

\mathrm{C_6H_{12}}

has two fewer H than hexane

\mathrm{C_6H_{14}}

even though it has no double bond. When ranking named compounds by hydrogen count, a 'cyclo' or a 'methylcyclo' costs 2 H, exactly like an '-ene'.

Functional Groups: Recognise, Name and Find Them in Biomolecules

Learn this subtopic in the notes

Naming compounds that contain a functional group

Name with a functional group

\text{branches} + \text{stem} + \text{locant} + \text{group ending}

example4-methylpentan-2-ol
locantthe carbon of the group; omitted for aldehydes and acids

The main functional groups and their families

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 molecules

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

Common traps

Amine or amide: look for the C=O next to the nitrogen

Both contain nitrogen. If the N is bonded directly to a C=O carbon, the group is an amide (as in paracetamol and in proteins); otherwise it is an amine. An amide is not an amine plus a ketone.

Ethers have no C=O at all

An ether is just C-O-C. Aldehydes, ketones, acids, esters and amides all contain C=O, whatever their common names (acetaldehyde, acetone, acetic acid, methyl acetate). In a list asking which molecule lacks C=O, the ether or the alcohol is the answer.

The OH of a COOH group is not an alcohol

In a carboxylic acid the OH is on the C=O carbon, and together they make one group. Listing 'alcohol' as well is double counting. An alcohol needs an OH on a carbon with no C=O.

Esters are named alcohol part first

\mathrm{CH_3COOCH_2CH_3}

the acid part is

\mathrm{CH_3COO}

(ethanoate) and the alcohol part is

\mathrm{CH_2CH_3}

(ethyl). The name is ethyl ethanoate. Reading the formula left to right and naming the first part first gives the wrong ester.

Glycogen is a homopolysaccharide stored by animals, not plants

Glycogen is a branched chain of glucose units only, so it is a homopolysaccharide, not a heteropolysaccharide and not a disaccharide. Animals store glycogen; plants store starch.

Amino acids are water-soluble ions, not oily molecules

Because each amino acid carries both an acidic and a basic group, it exists as a zwitterion in water. That makes it dissolve well in water and badly in hexane, and gives it both acid and base behaviour.

Isomerism: Structural Isomers, Cis/Trans and Chirality

Learn this subtopic in the notes

Chirality: chiral carbons and optical isomers

Maximum number of stereoisomers

N_{\max} = 2^{n}

$n$ number of chiral centres
$N_{\max}$ upper limit; symmetric molecules can have fewer

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

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

Common traps

Isomers need not have similar physical properties

The definition only requires the same molecular formula and a different structure. Ethanol boils at about 78 °C while its isomer dimethyl ether boils at about −24 °C. A statement that isomers must have similar properties is false.

A formula with one oxygen and one degree of unsaturation is not only a carbonyl compound

\mathrm{C_4H_8O}

can be butanal or butanone, but also an alcohol with a C=C (but-3-en-1-ol) or a ring (cyclobutanol), or a cyclic ether. Options that rule out the alcohol forget that a ring or C=C can supply the missing two hydrogens.

A C=C is needed for cis/trans, but it is not enough

Every alkene has a C=C, but only some have cis/trans isomers. If either double-bond carbon carries two identical groups, swapping sides gives the same molecule. Check both carbons separately.

Enantiomers have the same physical properties

Boiling point, melting point, density and solubility are identical for the two enantiomers. They differ only in the direction they rotate plane-polarised light and in how they interact with other chiral molecules, such as enzymes and receptors.

A carbon in a double bond cannot be a chiral centre

A chiral centre needs four different groups on four single bonds, so it must be sp³. A C=C or C=O carbon is joined to only three atoms and is never chiral.

Organic Reactions: Types, Oxidation, Esters and Combustion

Learn this subtopic in the notes

Esterification, hydrolysis and amide formation

Esterification

\mathrm{RCOOH + R'OH \rightleftharpoons RCOOR' + H_2O}

$\mathrm{RCOOH}$ the carboxylic acid; gives the -oate half of the name
$\mathrm{R'OH}$ the alcohol; gives the alkyl half of the name

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}

$x, y, z$ numbers of C, H and O atoms in the fuel ( $z = 0$ for a hydrocarbon)

The main types of organic reaction

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 alcohols

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

Common traps

Adding bromine to an alkene is also an oxidation of carbon

In ethene each carbon has oxidation number −2; in 1,2-dibromoethane each has −1. The carbons have been oxidised, even though no oxygen is involved. Adding

\mathrm{H_2}

to make ethane is the opposite: a reduction.

Condensation joins molecules; elimination splits one

Both release water. In condensation two molecules join into a bigger one (acid + alcohol to ester). In elimination a single molecule loses water and gains a C=C (ethanol to ethene).

A primary alcohol never gives a ketone

The carbon carrying the OH in a primary alcohol is at the end of the chain, so its C=O can only be an aldehyde, then an acid. Ketones come only from secondary alcohols, and ketones are not oxidised further.

Base hydrolysis gives a salt, not the free acid

Boiling an ester with NaOH gives the sodium carboxylate and the alcohol, and the reaction goes to completion. The free carboxylic acid appears only after acidifying, or in acid hydrolysis, which stays reversible.

Subtract the oxygen the fuel already contains

For alcohols, aldehydes and acids, part of the oxygen in the products comes from the fuel itself. Use

x + y/4 - z/2

; leaving out the

z/2

gives the answer for the hydrocarbon with the same C and H.

Physical Properties: Boiling Points and Solubility

Learn this subtopic in the notes

Boiling points of organic compounds: size, branching and hydrogen bonds

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 solvents

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

Common traps

Boiling breaks forces between molecules, not covalent bonds

When ethanol boils, its molecules separate but each one stays whole: the C-O and O-H bonds are not broken. Options that explain a boiling point by bond strength inside the molecule are wrong.

Branching lowers the boiling point even with the same formula

Isomers have the same molar mass but can boil at very different temperatures. The more branched isomer is more compact, touches its neighbours less, and boils lower.

A molecule does not need its own O-H to dissolve in water

Propanone and small esters have no O-H, yet they dissolve because water can hydrogen bond to their oxygen. The test is whether the molecule can hydrogen bond with water, not with itself.

Organic Chemistry formulas

How Carbon Bonds, and How We Write Organic Molecules

Molecular, condensed and skeletal formulas, and counting the hydrogens

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

Common traps

Hydrocarbons: Families, Benzene, Names and Unsaturation

IUPAC names for alkanes and alkenes

Degrees of unsaturation: rings and multiple bonds from a formula

Hydrocarbon families and their general formulas

Benzene and aromatic compounds

Common traps

Functional Groups: Recognise, Name and Find Them in Biomolecules

Naming compounds that contain a functional group

The main functional groups and their families

Functional groups in biological molecules

Common traps

Isomerism: Structural Isomers, Cis/Trans and Chirality

Chirality: chiral carbons and optical isomers

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

Common traps

Organic Reactions: Types, Oxidation, Esters and Combustion

Esterification, hydrolysis and amide formation

Combustion of organic compounds

The main types of organic reaction

Oxidation of primary, secondary and tertiary alcohols

Common traps

Physical Properties: Boiling Points and Solubility

Boiling points of organic compounds: size, branching and hydrogen bonds

Solubility of organic compounds in water and in non-polar solvents

Common traps

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