Introduction to Organic Chemistry

Section 1 of 17Types of formula

Types of formula

Empirical formula = the simplest whole-number ratio of elements in a compound, such as CH₂O for ethanoic acid

Molecular formula = the number and type of each atom in a molecule, such as C₂H₄O₂

General formula = a formula for a homologous series using letters and numbers, such as C_nH_2n+2 for alkanes

Structural formula = how atoms are bonded to each carbon in the molecule

Displayed formula = a 2D drawing of all atoms and bonds

Skeletal formula = a simplified displayed formula with the carbon-hydrogen bonds removed

Examples

• C₂H₄Cl₂ has empirical formula CH₂Cl, and C₄H₆ has empirical formula C₂H₃

Section 2 of 17Homologous series and functional groups

Homologous series and functional groups

Functional group = a specific atom or group of atoms that determines the physical and chemical properties of the molecule

Classification

• Classification molecules are classified by their dominant functional group

Hydrocarbons

• Hydrocarbons compounds of carbon and hydrogen only, with alkanes the simplest and having no functional group

Homologous series = a group of organic compounds with the same functional group, the same general formula and similar chemical properties

Pattern

• Pattern each extra carbon adds two hydrogens, so members differ by –CH₂–

Section 3 of 17Trends within a homologous series

• Physical properties
  • they change gradually, such as boiling point, melting point and density

Alkane	Carbons	Formula	Boiling point / °C	State at room temperature	Melting point / °C
Methane	1	CH4	-162	gas	-183
Ethane	2	C2H6	-89	gas	-172
Propane	3	C3H8	-42	gas	-188
Butane	4	C4H10	0	gas	-138
Pentane	5	C5H12	36	liquid	-130

Boiling point

• Boiling point it increases with molecular size, because each extra –CH₂– strengthens the intermolecular forces

Other series

• Other series similar trends are seen in other series

Section 4 of 17IUPAC nomenclature: stems

• Systematic name
  • the stem shows the number of carbons in the longest chain, based on the alkanes

Carbons	Alkane	Stem
1	methane CH4	meth-
2	ethane C2H6	eth-
3	propane C3H8	prop-
4	butane C4H10	but-
5	pentane C5H12	pent-
6	hexane C6H14	hex-
7	heptane C7H16	hept-
8	octane C8H18	oct-
9	nonane C9H20	non-
10	decane C10H22	dec-

Section 5 of 17IUPAC nomenclature: branches and groups

IUPAC nomenclature: branches and groups

Numbering

• Numbering number the longest chain from the end that gives the lowest possible numbers

Side chains

• Side chains they are named by adding -yl to the stem (alkyl groups) and shown in brackets in a structural formula

Multiples

• Multiples di-, tri- and tetra- show two, three and four identical groups

Punctuation

• Punctuation commas separate numbers and hyphens separate numbers and words

Alphabetical order

• Alphabetical order different alkyl groups are listed alphabetically, each with its carbon number

Examples

• 2,5,5-trimethylhex-2-ene: a 6-carbon chain, a double bond on carbon 2, and methyls on carbons 2, 5 and 5.
• Propanalthe aldehyde group is always carbon 1.
• 2-methylbutanal: a 4-carbon chain, methyl on carbon 2, aldehyde on carbon 1.
• Butanoic acidthe carboxylic acid group is carbon 1.
• 2-chlorobutane: chlorine on carbon 2.

Aliphatic

• Aliphatic straight or branched chains, including rings with no benzene ring

Section 6 of 17Homolytic and heterolytic fission

Homolytic fission = breaking a covalent bond so that each atom takes one electron, forming two radicals

Heterolytic fission = breaking a covalent bond so that the more electronegative atom takes both electrons, forming a negative and a positive ion

Syllabus note

• Syllabus note curly arrows are not required for free radical substitution

Section 7 of 17Free radicals and the free radical substitution mechanism

1

Free radical = a species with one or more unpaired electrons, shown with a dot, such as Cl•

2Free radical substitution

• Free radical substitution a reaction involving radicals in three steps

3

Initiation

• Covalent bonds break using ultraviolet light.
• Two free radicals form.

4

Propagation

• Radicals attack reactant molecules to form more radicals.
• These attack other molecules in turn, continuing the chain.

5

Termination

• Two free radicals react together to form a product molecule.

6Exam skill

• Exam skill write balanced equations for the steps, such as chlorination of methane at AS and substitution of arenes at A Level

Section 8 of 17Fundamentals of reaction mechanisms

1Difference from equations

• Difference from equations equations show amounts of reactants and products, while mechanisms show how the reaction occurs

2Curly arrows

• Curly arrows they show the movement of electrons in forming or breaking a covalent bond

Section 9 of 17Rules for curly arrows

1

The arrow must be double-headed.

2

It must start from a lone pair of electrons or a covalent bond.

3

It must end at an area of positive charge.

4

Include partial charges δ- and δ+, full charges and lone pairs where appropriate.

• Example
  • in an addition step, a lone pair on the bromide ion attacks a positive carbocation to form a new covalent bond

Section 10 of 17Types of organic reaction

Types of organic reaction

Addition

• Addition two reactants form one product, such as ethene and bromine forming 1,2-dibromoethane

Substitution

• Substitution an atom or group is replaced by another, such as bromoethane with hydroxide giving ethanol and bromide

Elimination

• Elimination a small molecule is removed from a larger one, such as ethanol with an acid catalyst giving ethene and water

AS mechanisms	A Level mechanisms
Nucleophilic substitution	Nucleophilic addition of carbonyls
Elimination of halogenoalkanes	Nucleophilic addition–elimination with water and alcohols
Electrophilic addition	Electrophilic substitution
Elimination of alcohols	Nucleophilic addition–elimination with ammonia and primary amines

Section 11 of 17Structural isomerism

Structural isomerism

Structural isomers = compounds with the same molecular formula but different structural formulae, such as propene and cyclopropane (both C₃H₆)

Chain isomerism

• Chain isomerism the longest chain differs because of branching
• Pentane and 2,2-dimethylpropane are both C₅H₁₂.
• Their longest chains are 5 and 3 carbons.

Positional isomerism

• Positional isomerism the functional group is on different carbons
• Butan-1-ol and butan-2-ol are both C₄H₁₀O.

Functional group isomerism

• Functional group isomerism different functional groups give very different chemical properties
• Butan-1-ol (alcohol) and ethoxyethane (ether) are both C₄H₁₀O.

Section 12 of 17Deducing isomers

Skill

• Skill deduce all possible isomers from a molecular formula

Test

• Test name each structure, since isomers have different names and bending a molecule through 90° does not make an isomer

C₃H₆Br₂

• No functional group isomerism, as Br is the only group.
• No chain isomerism, as the longest chain is 3.
• Positional isomerism occurs because the bromines can be on different carbons.

C₄H₁₀

• No functional group or positional isomerism.
• Chain isomerism gives butane and 2-methylbutane (2-methylpropane).

Section 13 of 17E/Z isomerism

Stereoisomers = compounds with the same atoms connected in the same order but arranged differently in space

Cause

• Cause restricted rotation about the planar C=C double bond

Condition

• Condition each C=C carbon must carry two different atoms or groups, so 2-methylpropene has no E/Z isomers

Z isomer

• Z isomer the higher-priority groups are on the same side of the double bond
  • (zusammen, together)

E isomer

• E isomer the higher-priority groups are on opposite sides
  • (entgegen, opposite)

Terms

• Terms geometric isomer is an outdated term, and cis/trans is superseded by E/Z and not required

Section 14 of 17CIP priority rules

• Rule
  • compare the atomic number of the first atom attached to each carbon of the C=C, and the higher atomic number has higher priority

1

Look at the two groups on the left carbon and find the higher priority.

2

Look at the two groups on the right carbon and find the higher priority.

3

If the higher-priority groups are on opposite sides, the isomer is E.

4

If they are on the same side, the isomer is Z.

Example

• 1-bromo-1-propen-2-ol has Br above H and O above C, so the isomer with Br and O on opposite sides is E and the isomer with both on the same side is Z

Naming

• E-2-bromo-1-chloroprop-1-ene has chlorine on carbon 1 and bromine on carbon 2 on opposite sides

Section 15 of 17E/Z when the first atoms are the same

• Next step
  • if the first atoms are equal, compare the second atoms attached

1

Ignore atoms identical on both groups, such as hydrogens.

2

Compare the first point of difference by atomic number.

3

CH₂Br outranks CH₃CH₂ because Br outranks C.

4

CH₂OH outranks CH₃ because O outranks H.

1Result

• Result when the two higher-priority groups are on the same side, the isomer is Z

2Drawing

• E-2-bromobut-2-ene has Br outranking CH₃ on one carbon and CH₃ outranking H on the other, with the higher-priority groups on opposite sides

Section 16 of 17Key terms

Term	Meaning
Homologous series	A group of compounds with the same functional group and general formula, differing by –CH2–.
Functional group	The atom or group of atoms that determines a compound’s characteristic reactions.
Free radical	A species with an unpaired electron.
Initiation	The step in which radicals are first formed.
Propagation	Chain steps in which radicals react and form more radicals.
Termination	A step in which two radicals combine.

Term	Meaning
Structural isomers	Compounds with the same molecular formula but different structural formulae.
Stereoisomers	Compounds with the same structural formula but different arrangements in space.
E/Z isomerism	Stereoisomerism about a C=C bond using CIP priorities.
CIP rules	Priority rules based on atomic number.
Positional isomers	Isomers with the functional group in different positions.
Chain isomers	Isomers with different carbon chain arrangements.

Section 17 of 17Exam technique

1

Writing the empirical formula for ionic compounds as different from the molecular formula — for these, they are the same.

2

Numbering the chain from the wrong end — the lowest numbers must be obtained.

3

Writing side chains in numerical order — list them alphabetically.

4

Saying heterolytic fission forms radicals — it forms ions.

1

Naming isomers from drawings that look different — compare names, not drawings.

2

Using cis/trans in exam answers — use E/Z.

3

Using the first atoms only when they are equal — compare the next atoms.

4

Saying E means entgegen is together — E is opposite and Z is together.