Notes · ChemistryUK · A-Levels

Introduction to organic chemistry

Organic chemistry is the chemistry of carbon compounds, and this chapter sets up the language and tools used throughout it. It covers functional groups and homologous series, IUPAC nomenclature and the different ways of representing molecules, structural and E-Z isomerism, and the conventions of reaction mechanisms - curly arrows, bond fission and the classification of reagents as nucleophiles, electrophiles or radicals.

5 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 4

T·111111 / 18

Exam profile

AO1 · Recall the naming rules, functional groups, formula types and types of isomerismAO2 · Name and draw organic structures, identify isomers and use curly-arrow conventionsAO3 · Deduce structures from formulae and classify reagents and bond-fission types

Operators:statenamedrawdeduceclassifyexplain

basic level

AS-Level requires nomenclature, functional groups, the formula types, structural and E-Z isomerism, and the meaning of curly arrows and reagent types.

higher level

The full A-Level uses these conventions in every organic mechanism that follows, and extends isomerism to optical isomerism in the later organic topic.

Depth

Text

Contents · 5 sections

Introduction to organic chemistry

Functional groups, homologous series and general formulae#

FoundationAQA 7405 3.3.1.1DfE GCE Chemistry - organic chemistry

Common functional groups and series

Fig. 1Each homologous series shares a functional group and a general formula, with members differing by CH2.

A functional group is the atom or group of atoms in a molecule that is responsible for its characteristic reactions - the C=C double bond of an alkene, the -OH of an alcohol, the -COOH of a carboxylic acid. Because chemistry is concentrated at the functional group, molecules with the same functional group behave in similar ways, and organic chemistry is organised around these groups rather than around individual compounds.

A homologous series is a family of compounds with the same functional group and the same general formula, in which each member differs from the next by

\text{CH}_2

. The alkanes (

\text{C}_n\text{H}_{2n+2}

), alkenes (

\text{C}_n\text{H}_{2n}

) and alcohols (

\text{C}_n\text{H}_{2n+1}\text{OH}

) are examples. Members of a series show a gradual trend in physical properties (such as boiling point rising with chain length) and very similar chemical properties, because they share the functional group.

The general formula lets you write the molecular formula of any member of a series from the number of carbon atoms. For the alkanes,

\text{C}_n\text{H}_{2n+2}

gives

\text{CH}_4

for

n=1

\text{C}_2\text{H}_6

for

n=2

, and so on. Knowing the general formulae of the common series is a quick way to check a molecular formula or to deduce a possible structure.

Because the functional group controls reactivity, recognising it is the first step in predicting how a molecule will react. When you meet an unfamiliar organic molecule, identify its functional group first: this tells you which homologous series it belongs to, which reactions it is likely to undergo, and which naming rules apply. This 'group-first' habit organises the whole of organic chemistry.

\text{alkanes: } \text{C}_n\text{H}_{2n+2}, \qquad \text{alkenes: } \text{C}_n\text{H}_{2n}

General formulae

Substitute the number of carbons n to get the molecular formula of any member.

Worked example

Using a general formula

Write the molecular formula of the alkane and the alkene that each contain five carbon atoms.

Alkane
CnH2n+2 with n = 5 gives C5H(2x5+2) = C5H12 (pentane).
Alkene
CnH2n with n = 5 gives C5H10 (a pentene).
Compare
The alkene has two fewer hydrogens because of its C=C double bond.

Result: Pentane is C5H12 and pentene is C5H10.

Typical mistakes

Confusing the general formula of alkanes (CnH2n+2) with that of alkenes (CnH2n).
Thinking members of a homologous series differ chemically - they share the functional group and react similarly.

Active revision

State the general formula of the alcohols and use it to give the molecular formula of the alcohol with four carbon atoms.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Chemistry 7405 specification (AQA)

Nomenclature and formulae of organic compounds#

StandardAQA 7405 3.3.1.1DfE GCE Chemistry - nomenclature

Skeletal formula of 2-methylbutane

Fig. 2In a skeletal formula each vertex and line-end is a carbon; hydrogens on carbon are omitted. This is 2-methylbutane.

IUPAC nomenclature names a molecule systematically so that the name specifies the structure exactly. The stem gives the number of carbon atoms in the longest chain (meth-, eth-, prop-, but-, pent-, hex- for one to six), the suffix gives the main functional group (-ane, -ene, -ol, -al, -one, -oic acid), and prefixes name any side chains or substituents (methyl-, chloro-). Locants (numbers) show where the groups are attached, chosen to give the lowest possible set of numbers.

The steps are: find the longest carbon chain containing the functional group (this gives the stem and suffix), number the chain from the end that gives the functional group and substituents the lowest locants, and name and number each substituent, listing them alphabetically. So

\text{CH}_3\text{CH(CH}_3)\text{CH}_2\text{CH}_3

is 2-methylbutane, and

\text{CH}_3\text{CH(OH)CH}_3

is propan-2-ol.

Organic molecules can be represented in several ways, each with a purpose. The empirical formula gives the simplest ratio of atoms; the molecular formula the actual number of each atom; the structural formula shows the arrangement of groups in a condensed line (such as

\text{CH}_3\text{CH}_2\text{OH}

); the displayed formula shows every atom and every bond; and the skeletal formula shows the carbon skeleton as lines, with carbons at each vertex and end and hydrogens on carbon left out. Skeletal formulae are the quickest to draw for larger molecules.

Being fluent in converting between these representations, and between a name and a structure, is essential. A displayed formula makes bonding clear but is slow to draw; a skeletal formula is fast and clear for ring and chain systems; a structural formula is compact for writing equations. When a question gives a name, draw the structure to work with it; when it gives a structure, name it carefully following the numbering rules.

Worked example

Naming a branched molecule

Name the molecule CH3CH(CH3)CH2CH3.

Find the longest chain
The longest chain has four carbons, so the stem is but- and (all single bonds) the suffix is -ane: butane.
Number for the lowest locant
Numbering from the end nearest the branch places the methyl group on carbon 2.
Assemble the name
A methyl substituent on C2 of butane gives 2-methylbutane.

Result: The molecule is 2-methylbutane.

Typical mistakes

Numbering the chain from the wrong end, giving higher locants than necessary.
Missing the longest carbon chain (which may not be drawn in a straight line).

Active revision

Give the IUPAC name of $\text{CH}_3\text{CH}_2\text{CH(CH}_3)\text{CH}_2\text{OH}$ and draw its skeletal formula.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

Structural isomerism#

StandardAQA 7405 3.3.1.2DfE GCE Chemistry - isomerism

Types of isomerism

Fig. 3Isomers divide into structural (different connectivity) and stereoisomers (same connectivity, different arrangement in space).

Isomers are compounds with the same molecular formula but a different arrangement of atoms. Structural (constitutional) isomers have their atoms joined together in a different order, and there are three kinds. Chain isomers differ in the branching of the carbon skeleton (butane and 2-methylpropane both have the formula

\text{C}_4\text{H}_{10}

). Position isomers have the same functional group in a different position on the chain (propan-1-ol and propan-2-ol). Functional-group isomers have a different functional group altogether (ethanol, an alcohol, and methoxymethane, an ether, are both

\text{C}_2\text{H}_6\text{O}

Recognising and drawing structural isomers is a routine skill. Given a molecular formula, you systematically vary the skeleton (chain isomers), then the position of the functional group (position isomers), then consider whether a different functional group is possible (functional-group isomers). Care is needed not to draw the same isomer twice in a different orientation - two structures that can be superimposed by rotation are the same compound, not isomers.

Structural isomers often have markedly different physical properties. Branched-chain isomers have lower boiling points than their straight-chain counterparts, because branching reduces the surface area of contact between molecules and so weakens the van der Waals forces; 2-methylpropane boils lower than butane. Isomers may also differ chemically if their functional groups differ, as ethanol and methoxymethane do.

Structural isomerism is distinct from stereoisomerism (covered next): in structural isomers the atoms are connected in a different order, whereas in stereoisomers the atoms are connected in the same order but arranged differently in space. Keeping the two categories separate - different connectivity versus different spatial arrangement - is important when a question asks for a particular type of isomer.

Worked example

Isomers of C3H8O

Draw and classify three isomers of C3H8O.

Position isomers
Propan-1-ol (CH3CH2CH2OH) and propan-2-ol (CH3CH(OH)CH3) differ only in the position of the -OH group.
Functional-group isomer
Methoxyethane (CH3OCH2CH3), an ether, has the same formula but a different functional group.
Classify
The two alcohols are position isomers of each other; the ether is a functional-group isomer of both.

Result: Propan-1-ol and propan-2-ol (position isomers) and methoxyethane (a functional-group isomer) all have the formula C3H8O.

Typical mistakes

Drawing the same isomer twice in a different orientation and counting it as two.
Confusing structural isomerism (different connectivity) with stereoisomerism (same connectivity).

Active revision

Draw and name the two chain isomers of $\text{C}_4\text{H}_{10}$ and explain which has the higher boiling point.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

E-Z stereoisomerism#

StandardAQA 7405 3.3.1.3DfE GCE Chemistry - E-Z isomerism

E and Z but-2-ene

Fig. 4Restricted rotation about C=C locks the methyl groups either on the same side (Z) or on opposite sides (E).

Stereoisomers have the same structural formula (the same atoms joined in the same order) but a different arrangement of the atoms in space. E-Z isomerism (a form of cis-trans isomerism) arises about a carbon-carbon double bond. Because the

\pi

bond prevents rotation about the C=C double bond, groups on either end of the bond are locked in position, so two different spatial arrangements are possible and cannot interconvert without breaking the

\pi

bond.

For E-Z isomerism to exist, each carbon of the double bond must carry two different groups. If one carbon has two identical groups (as in

\text{CH}_2{=}\text{CHCl}

, where one carbon has two hydrogens), there is only one arrangement and no E-Z isomers. But-2-ene,

\text{CH}_3\text{CH}{=}\text{CHCH}_3

, does show E-Z isomerism because each double-bond carbon carries a methyl group and a hydrogen.

The isomers are named by the CIP priority rules. On each carbon of the double bond, the group of higher priority is the one whose attached atom has the higher atomic number. If the two higher-priority groups are on the same side of the double bond, the isomer is Z (from the German zusammen, together); if on opposite sides, it is E (entgegen, opposite). For simple cases where the higher-priority groups are the larger carbon groups, Z corresponds to the older label cis and E to trans.

E-Z isomers can have different physical and even biological properties, because their shapes differ. The cis (Z) isomer of but-2-ene has a slightly higher boiling point than the trans (E) isomer, and in nature the geometry of double bonds is often crucial - for example in the difference between cis and trans fats. When drawing E-Z isomers, show the double bond flat and place the substituents clearly on the same side (Z) or opposite sides (E).

Worked example

Deciding whether E-Z isomerism exists

State, with reasons, whether 1,1-dichloroethene (CCl2=CH2) and 1,2-dichloroethene (CHCl=CHCl) show E-Z isomerism.

1,1-dichloroethene
One carbon carries two chlorines (identical groups) and the other two hydrogens, so there is only one arrangement - no E-Z isomerism.
1,2-dichloroethene
Each carbon carries one Cl and one H (two different groups), so E-Z isomerism is possible.
Assign
The Z isomer has the two chlorines on the same side; the E isomer has them on opposite sides.

Result: 1,1-dichloroethene has no E-Z isomers, but 1,2-dichloroethene has an E and a Z isomer.

Typical mistakes

Claiming E-Z isomerism when one double-bond carbon carries two identical groups (then there is only one form).
Confusing E with Z - Z has the higher-priority groups on the same side.

Active revision

Explain why but-2-ene shows E-Z isomerism but but-1-ene ( $\text{CH}_2{=}\text{CHCH}_2\text{CH}_3$ ) does not.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

Reaction mechanisms and reagent types#

StandardAQA 7405 3.3.1.1DfE GCE Chemistry - reaction mechanisms

Homolytic and heterolytic fission

Fig. 5Homolytic fission (fishhooks) gives two radicals; heterolytic fission (a full curly arrow) gives a cation and an anion.

A reaction mechanism shows how a reaction happens step by step, using curly arrows to track the movement of electrons. A curly arrow always shows the movement of a pair of electrons: it starts at a bond or a lone pair (the source of the electrons) and points to where the electron pair goes (forming a new bond or a lone pair). A double-headed arrow moves a pair of electrons; a single-headed 'fishhook' arrow moves a single electron in radical reactions.

Covalent bonds break in two ways. In homolytic fission the bond breaks evenly, each atom taking one of the shared electrons, forming two radicals - species with an unpaired electron, shown by a dot. In heterolytic fission the bond breaks unevenly, both electrons going to one atom, forming a positive ion (cation) and a negative ion (anion). Homolytic fission is typical of non-polar bonds under UV light; heterolytic fission of polar bonds.

Reagents that attack in polar mechanisms are classified by what they seek. A nucleophile is an electron-pair donor - a species with a lone pair that is attracted to an electron-poor (positive) centre; examples are

\text{OH}^-

\text{CN}^-

\text{NH}_3

and water. An electrophile is an electron-pair acceptor - a species attracted to an electron-rich (negative) centre such as a C=C double bond; examples are

\text{H}^+

\text{NO}_2^+

and the polarised or induced-dipole halogen molecules. A radical has an unpaired electron and reacts by homolytic pathways.

Matching the mechanism to the reagent and substrate is the organising idea of the reactions to come: a nucleophile attacks an electron-poor carbon (nucleophilic substitution or addition), an electrophile attacks an electron-rich double bond or ring (electrophilic addition or substitution), and radicals drive substitution of alkanes under UV. When you write any mechanism, draw the curly arrows correctly - from a lone pair or bond to where the electrons go - and label the type of reagent involved.

Worked example

Classifying a reagent

Explain why the hydroxide ion (OH-) is a nucleophile and predict the type of carbon atom it will attack.

Identify the electron pair
The hydroxide ion has lone pairs of electrons and a negative charge, making it electron-rich.
Classify
Because it can donate a lone pair to form a new bond, it is a nucleophile (an electron-pair donor).
Predict the target
It is attracted to an electron-poor (delta-positive) carbon, such as the carbon of a polar C-Br bond in a halogenoalkane.

Result: OH- is a nucleophile (electron-pair donor) and attacks the delta-positive carbon of a polar bond.

Typical mistakes

Drawing a curly arrow from an atom rather than from a bond or a lone pair.
Confusing nucleophiles (electron-pair donors, attack positive centres) with electrophiles (electron-pair acceptors, attack negative centres).

Active revision

Classify each of $\text{OH}^-$ , $\text{NO}_2^+$ and a chlorine radical as a nucleophile, electrophile or radical, and state the type of centre each attacks.

Active recall

Recall the key points — then reveal.

Sources: AQA A-level Chemistry 7405 specification (AQA)

References & sources

Sources

Department for Education

GCE AS and A level subject content for the sciences

AQA

AQA A-level Chemistry 7405 specification

Introduction to organic chemistry

5 sections~14 min reading time3 competenciesLevel Foundation 1 · Standard 4

Introduction to organic chemistry

Key points

Using a general formula

Key points

Naming a branched molecule

Key points

Isomers of C3H8O

Key points

Deciding whether E-Z isomerism exists

Key points

Classifying a reagent

Sources

Introduction to organic chemistry

Key points

Using a general formula

Key points

Naming a branched molecule

Key points

Isomers of C3H8O

Key points

Deciding whether E-Z isomerism exists

Key points

Classifying a reagent

Sources