Learners' AlleyDesigned by Khow Willard

Introduction to Organic Chemistry

1

Formulae of Organic Compounds

Carbon forms many compounds: strong C–C bonds allow long chains and rings (catenation), C is tetravalent, allowing branching and multiple bonds.

Formula	Shows	Butanoic acid
Empirical	simplest ratio of atoms of each element	C₂H₄O
Molecular	actual number of atoms of each element in one molecule	C₄H₈O₂
Structural	how the atoms are joined, with minimal detail	CH₃CH₂CH₂CO₂H
Displayed	all atoms and all bonds. Bent (–O–H) and trigonal planar (C=O) groups must be drawn out with their shapes
Skeletal	carbon skeleton and functional groups only, C and H on C not shown
Stereochemical	3D arrangement: wedge = out of the plane, dash = into the plane

2

Terminology

DefinitionHydrocarbon: a compound containing carbon and hydrogen only

Functional group: an atom or group of atoms responsible for the chemical reactions of a compound
Alkyl group, R: C_nH_2n+1, e.g. CH₃– methyl, C₂H₅– ethyl
Saturated: only single bonds. Unsaturated: contains multiple bonds. Each π bond removes 2 H.
Homologous series: a series of compounds with the same functional group, each member differing by –CH₂–. Its members have:
- similar chemical reactions
- similar methods of preparation
- M_r increasing by 14 from one member to the next
- a gradual change in physical properties

2.1Degree of substitution

A carbon bonded to 1, 2, 3 or 4 other carbons is primary (1°), secondary (2°), tertiary (3°) or quaternary (4°).

2,2,4-trimethylpentane

50Topic 11.1 · Introduction to Organic Chemistry

2

Terminology (continued)

2.2Classes of compounds, in order of priority

Class	General	Group	Suffix	Prefix
carboxylic acid	RCO₂H	–CO₂H	-oic acid	–
ester	RCO₂R′	–CO₂–	-oate	alkoxy- carbonyl-
acyl chloride	RCOCl	–COCl	-oyl chloride	chloro- carbonyl-
amide	RCONH₂	–CONH–	-amide	–
nitrile	RCN	–C≡N	-nitrile	cyano-
aldehyde	RCHO	–CHO	-al	–
ketone	RCOR′	C=O	-one	–

Class	General	Group	Suffix	Prefix
alcohol	ROH	–OH	-ol	hydroxy-
phenol	ArOH	–OH	phenol	hydroxy-
amine	RNH₂, R₂NH, R₃N	–N	-amine	amino-
alkene	C_nH_2n	C=C	-ene	–
alkane	C_nH_2n+2	–	-ane	alkyl-
arene	C₆H₅–R	C₆H₅–	-benzene	phenyl-
halogeno- alkane	RX	–X	–	chloro-, bromo-, iodo-
amino acid	RCH(NH₂) CO₂H	–NH₂, –CO₂H	-oic acid	amino-

The highest group in the order gives the suffix, and every other group becomes a prefix: HOCH₂CH₂CHO is 3-hydroxypropanal, H₂NCH₂CO₂H is aminoethanoic acid. Halogens and alkyl groups are always prefixes, and C=C stays in the stem, as in but-3-en-1-ol.

An ester is named alkyl first, as in ethyl ethanoate: R′ (on O) must be an alkyl group, R may be H. The carbon of –C≡N and of –CHO counts in the stem, as in ethanenitrile, CH₃CN.

2.3Naming organic compounds

No. of C	1	2	3	4	5	6	7	8	9	10
Stem	meth	eth	prop	but	pent	hex	hept	oct	non	dec

Steps

Find the longest carbon chain that contains the functional group: this gives the stem
Number the chain from the end that gives the functional group the lowest number
Name the substituents (methyl, chloro …) as prefixes with their positions, in alphabetical order (di-, tri- for repeats)
Add the suffix of the highest-priority functional group (table 2.2) with its position

Example: CH₃CH(CH₃)CH₂OH

Longest chain with the –OH: 3 C → prop
–OH on C1 → propan-1-ol
–CH₃ on C2 → 2-methylpropan-1-ol

3

Types of Organic Reactions

Type	What happens	Example
Addition	two molecules combine to form one product, π bond broken	CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Substitution	an atom or group is replaced by another	CH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻
Elimination	atoms removed from two adjacent atoms to form a π bond	CH₃CH₂OH → CH₂=CH₂ + H₂O
Condensation	two molecules join with loss of a small molecule (H₂O, HCl)	RCO₂H + R′OH ⇌ RCO₂R′ + H₂O
Hydrolysis	breaking of a bond by reaction with water	RCO₂R′ + H₂O ⇌ RCO₂H + R′OH
Oxidation	gain of O or loss of H, [O]	CH₃CH₂OH + 2[O] → CH₃CO₂H + H₂O
Reduction	gain of H or loss of O, [H]	CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃

Topic 11.1 · Introduction to Organic Chemistry51

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4

Hybridisation

DefinitionHybridisation: the mixing of atomic orbitals in various combinations to give a set of hybrid orbitals of equivalent energy for covalent bonding with other atoms.

Why hybridise? In its ground state C has only two unpaired electrons, yet in CH₄ it forms four identical bonds at 109.5°. So: one 2s electron is promoted, then the orbitals are mixed.

1. ground state C

→

2. promote one 2s e⁻

→

3. mix: sp³, sp² or sp

sp³

2s + three 2p mixed → 4 sp³ orbitals
tetrahedral, 109.5°, 4σ, 25% s
e.g. CH₄, ethane

sp²

2s + two 2p mixed → 3 sp² + 1 p
trigonal planar, 120°, 3σ + 1π, 33% s
e.g. ethene, benzene

sp

2s + one 2p mixed → 2 sp + 2 p
linear, 180°, 2σ + 2π, 50% s
e.g. ethyne, CO₂

Each hybrid orbital has one large and one small lobe, the large lobe is used in bonding. Hybrid orbitals form σ bonds, unhybridised p orbitals form π bonds.

4.1Hybridisation of each carbon atom

Count the regions of electron density around the C atom (a double or triple bond counts as one region):

Regions around C	Hybridisation	Shape
4 (all single bonds)	sp³	tetrahedral
3 (one C=C or C=O)	sp²	trigonal planar
2 (C≡C, or two C=C)	sp	linear

Every single bond is one σ bond
A double bond is 1σ + 1π, a triple bond is 1σ + 2π

Worked exampleFor pent-3-en-1-yne, CH₃CH=CHC≡CH, state the hybridisation of each carbon atom and the number of σ and π bonds in one molecule.

C5 has 4 single bonds → sp³, C4 and C3 each have one C=C → sp², C2 and C1 each have one C≡C → sp
σ bonds: 6 C–H + 4 C–C = 10
π bonds: 1 in C=C + 2 in C≡C = 3

52Topic 11.1 · Introduction to Organic Chemistry

5

Shapes and Bonding in Organic Molecules

Ethane: each C is sp³, tetrahedral, 109.5°, 4σ bonds

C–H: head-on overlap of an sp³ orbital of C with the 1s orbital of H
C–C: head-on overlap of an sp³ orbital of each C

Ethene: each C is sp², trigonal planar, 120°, 3σ + 1π

σ bonds: head-on overlap of sp² orbitals (and H 1s)
π bond: side-on overlap of the unhybridised 2p orbital of each C, above and below the plane

Ethyne: each C is sp, linear, 180°, 2σ + 2π

σ bonds: head-on overlap of sp orbitals (and H 1s)
2 π bonds: side-on overlap of the 2p_y orbitals and of the 2p_z orbitals, at right angles to each other

Benzene: each C is sp², 120°, planar, the delocalised π system is covered in Arenes.

σ bond: head-on overlap of hybrid orbitals (C–C) or of a hybrid orbital with H 1s (C–H)
π bond: side-on overlap of unhybridised p orbitals, electron density above and below the plane

Why is C–H in ethene stronger than in ethane?

sp² orbital has more s character than sp³
→ smaller orbital, electrons closer to the nucleus
→ more effective overlap, shorter and stronger bond

6

Reagents and Bond Fission

6.1Types of reagent

Reagent	Definition	Examples	Attacks
Nucleophile (Lewis base)	an electron pair donor, with a lone pair of electrons	OH⁻, CN⁻, Cl⁻, NH₃, H₂O, ROH	an electron-poor centre, e.g. a C^δ+ atom bonded to a halogen
Electrophile (Lewis acid)	an electron pair acceptor, electron deficient	H⁺, NO₂⁺, Br^δ+–Br^δ−, H^δ+–Cl	an electron-rich centre, e.g. C=C or a benzene ring
Free radical	a species with an unpaired electron	Cl•, Br•, •CH₃	C–H bonds, e.g. in alkanes

NoteA free radical is electron poor but is not an electrophile: an electrophile carries a partial or full positive charge.

Topic 11.1 · Introduction to Organic Chemistry53

Learners' AlleyDesigned by Khow Willard

6

Reagents and Bond Fission (continued)

6.2Homolytic fission

The two bonding electrons are shared equally: each atom takes one → two free radicals.

6.3Heterolytic fission

The more electronegative atom takes both electrons → a cation and an anion. C⁺ is a carbocation, C⁻ is a carbanion.

6.4Drawing curly arrows

Where it starts and endsAn arrow starts at a bond or a lone pair and points to where the electrons go

Which way electrons flowFrom electron-rich to electron-poor sites

The arrowheadFull: a pair of electrons moves. Half (fish-hook): one electron moves

What else to showδ+ / δ−, lone pairs and charges on the species involved

7

Naming a Mechanism

Answer structuremechanism name = attacking species + type of reaction

Look at the first step: what attacks the organic molecule? (nucleophile, electrophile or free radical)
Compare the starting reactant with the final product (ignore intermediates): is a group replaced (substitution) or are two molecules combined (addition)?

Reaction	Attacked by	Mechanism
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br	Br^δ+ attacks electron-rich C=C	electrophilic addition
CH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻	OH⁻ attacks C^δ+	nucleophilic substitution
CH₄ + Cl₂ → CH₃Cl + HCl (UV)	Cl• attacks C–H	free radical substitution
C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃)	Br⁺ attacks the benzene ring	electrophilic substitution

54Topic 11.1 · Introduction to Organic Chemistry

8

Factors Affecting Reactivity

8.1Inductive effect

Electron-donating: alkyl groups, increasing with size: –CH₃ < –C₂H₅ < –CH(CH₃)₂ < –C(CH₃)₃
Electron-withdrawing: groups more electronegative than H, e.g. –NO₂, –Cl, –Br, –I
The effect is stronger with more groups, and weaker the further the group is from the charge

cation (+)

anion (−)

electron-donating

charge dispersed → stabilised

charge intensified → destabilised

electron-withdrawing

charge intensified → destabilised

charge dispersed → stabilised

8.2Stability of carbocations

Why is a 3° carbocation more stable than a 1°?

Alkyl groups are electron-donating
→ three alkyl groups disperse the positive charge on C⁺ to a greater extent than one

8.3Stability of anions and acid strength

Arrange in increasing K_a:

–Cl is electron-withdrawing: it disperses the negative charge on the carboxylate ion
The closer Cl is to –CO₂⁻, the stronger the effect → more stable anion
→ the acid dissociates more readily → larger K_a

8.4Delocalisation

When three or more adjacent p orbitals overlap, the π electrons spread over a larger region → the molecule gains extra stability, e.g. benzene.

8.5Steric hindrance

Bulky groups near the reacting site get in the way of the attacking species and slow the reaction. E.g. OH⁻ must attack the C of C–Br from the side opposite Br: easy in CH₃Br with its small H atoms, but the three bulky benzene rings in (C₆H₅)₃CBr block it.

Topic 11.1 · Introduction to Organic Chemistry55

Learners' AlleyDesigned by Khow Willard

9

Isomerism

Isomers: compounds with the same molecular formula but a different arrangement of atoms.

DefinitionConstitutional (structural) isomers: molecules that have the same molecular formula but different structural formula. Types: chain, position and functional group.

DefinitionStereoisomers: compounds that have the same molecular and structural formula, but differ in the spatial arrangement of their atoms. Types: cis–trans isomers and enantiomers.

9.1Constitutional (structural) isomerism

Type	The isomers differ in	Example
Chain	the carbon skeleton (branching)	butane2-methylpropane
Position	the position of the same functional group	propan-1-olpropan-2-ol
Functional group	the functional group, e.g. an aldehyde and a ketone	propanalpropanone

Each pair has one molecular formula: C₄H₁₀, C₃H₈O and C₃H₆O.

9.2Cis–trans isomerism

Arises from restricted rotation about the C=C bond: rotation would break the π bond, formed by side-on overlap of p orbitals
Each C of the C=C must have two different groups attached to it
cis: the groups are on the same side of the C=C. trans: they are on opposite sides.

Physical properties of cis and trans isomers are in Topic 11.3.

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9

Isomerism (continued)

9.3Enantiomerism

Chiral centre: a carbon atom bonded to four different atoms or groups, marked *
Chiral molecule: a molecule that is not superimposable on its mirror image
Enantiomers: a pair of molecules that are non-superimposable mirror images of each other
A molecule with a plane of symmetry is superimposable on its mirror image, so it is not chiral, even if it has chiral centres

Exam tipDraw a pair of enantiomers as mirror images on either side of a mirror plane, with two bonds in the plane, one wedge and one dash.

9.4Properties of enantiomers

Property	Enantiomers are
Physical	identical, except the direction in which they rotate plane-polarised light (equal extent, opposite directions)
Chemical	identical, except in their interactions with another chiral molecule, e.g. an enzyme
Biological	often different, e.g. in drug action, one enantiomer may treat the illness while the other is inactive or harmful

Optically active: a sample that rotates plane-polarised light. It contains chiral molecules.
Racemic mixture: equal proportions of both enantiomers. As each enantiomer rotates plane-polarised light to equal extents but in opposite directions, optical activity cancels out in the racemic mixture.

9.5Deducing isomers

Steps

Constitutional: vary the chain, the position or the functional group
Cis–trans: look for a C=C with two different groups on each C
Enantiomers: look for carbons with four different groups, the chiral centres
Count: stereoisomers = 2ⁿ, n = chiral centres + C=C showing cis–trans (fewer if there is a plane of symmetry)

Worked example: pent-3-en-2-ol CH₃CH=CHCH(OH)CH₃ has one chiral centre and one C=C that shows cis–trans isomerism. n = 1 + 1 = 2, so 2² = 4 stereoisomers.

Exam tipTo find chiral centres in a skeletal formula, ignore the bends and the ends, as they cannot carry four different groups. Only look at the intersections.

Topic 11.1 · Introduction to Organic Chemistry57