Introduction
to Organic
Chemistry

H2 Chemistry (9476), JC1

By the end of this topic, you can answer

1

Ethanol and methoxymethane share the formula C2H6O. How do you draw and name each so nobody confuses them?

2

Why do all alcohols, from methanol to octanol, react in the same way?

3

Is turning ethanol into ethene an addition, a substitution or an elimination? How can you tell?

4

Why is ethene flat, while ethane is three-dimensional?

5

When a bond breaks, where do its electrons go, and how do curly arrows show it?

6

Why is chloroethanoic acid a stronger acid than ethanoic acid?

7

How can two molecules with the same atoms, bonded in the same order, act completely differently in the body?

Recall: what you know already

1Name the first four alkanes.

Methane, ethane, propane, butane.[1]

2From Chemical Bonding: how many σ and π bonds are in a C=C double bond?

One σ bond and one π bond.[1]

3From Chemical Bonding: what is the shape and bond angle around C in CH₄?

Tetrahedral, 109.5°.[1]
1

Formulae of Organic Compounds

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

FormulaShowsButanoic acid
Empiricalsimplest ratio of atoms of each elementC₂H₄O
Molecularactual number of atoms of each element in one moleculeC₄H₈O₂
Structuralhow the atoms are joined, with minimal detailCH₃CH₂CH₂CO₂H
1

Formulae of Organic Compounds

FormulaShowsButanoic acid
Displayedall atoms and all bonds
Skeletalcarbon skeleton and functional groups only, C and H on C not shown
Stereochemical3D 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: CnH2n+1, e.g. CH₃– methyl, C₂H₅– ethyl
  • Saturated: only single bonds. Unsaturated: contains multiple bonds. Each π bond removes 2 H.
2

Terminology

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
  • Mr increasing by 14 from one member to the next
  • a gradual change in physical properties

Your turnButan-1-ol has Mr 74. What is the Mr of pentan-1-ol? Why do they react in the same way?

88 (one more CH₂, +14).[1]
Both have the same functional group, –OH, which decides the reactions.[1]
2

Terminology

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

2

Terminology

2.2Classes of compounds, in order of priority

ClassGeneralGroupSuffixPrefix
carboxylic acidRCO₂H–CO₂H-oic acid–
esterRCO₂R′–CO₂–-oate–
acyl chlorideRCOCl–COCl-oyl chloride–
amideRCONH₂–CONH–-amide–
nitrileRCN–C≡N-nitrilecyano-
aldehydeRCHO–CHO-al–
ketoneRCOR′C=O-one–

An ester is named alkyl first, as in ethyl ethanoate: R′ (on O) must be an alkyl group, R may be H.

2

Terminology

2.2Classes of compounds, in order of priority (continued)

ClassGeneralGroupSuffixPrefix
alcoholROH–OH-olhydroxy-
phenolArOH–OH on ringphenolhydroxy-
amineRNH₂, R₂NH, R₃N–N-amineamino-
alkeneCnH2nC=C-ene–
alkaneCnH2n+2–-anealkyl-, e.g. methyl-
areneC₆H₅–Rbenzene ring-benzenephenyl-
halogenoalkaneRX–X–chloro-, bromo-, iodo-
amino acidRCH(NH₂)CO₂H–NH₂, –CO₂H-oic acidamino-
2

Terminology

2.3Naming organic compounds

No. of C12345678910
Stemmethethpropbutpenthexheptoctnondec
  1. Find the longest carbon chain that contains the functional group: this gives the stem
  2. Number the chain from the end that gives the functional group the lowest number
  3. Name the substituents as prefixes with their positions, in alphabetical order (di-, tri- for repeats)
  4. Add the suffix of the highest-priority group with its position
ExampleCH₃CH(CH₃)CH₂OH: the longest chain with the –OH has 3 C, so prop. The –OH on C1 gives propan-1-ol. The –CH₃ on C2 gives 2-methylpropan-1-ol.
Two groups: HOCH₂CH₂CHO. The aldehyde outranks the alcohol, so it takes the suffix: 3-hydroxypropanal.
3

Types of Organic Reactions

TypeWhat happensExample
Additiontwo molecules combine to form one product, π bond brokenCH₂=CH₂ + Br₂ → CH₂BrCH₂Br
Substitutionan atom or group is replaced by anotherCH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻
Eliminationatoms removed from two adjacent atoms to form a π bondCH₃CH₂OH → CH₂=CH₂ + H₂O
Condensationtwo molecules join with loss of a small molecule (H₂O, HCl)RCO₂H + R′OH ⇌ RCO₂R′ + H₂O
Hydrolysisbreaking of a bond by reaction with waterRCO₂R′ + H₂O ⇌ RCO₂H + R′OH
Oxidationgain of O or loss of H, [O]CH₃CH₂OH + 2[O] → CH₃CO₂H + H₂O
Reductiongain of H or loss of O, [H]CH₃COCH₃ + 2[H] → CH₃CH(OH)CH₃
3

Types of Organic Reactions

Quick checkCH₃CH₂Br + NaOH (in ethanol, heat) → CH₂=CH₂ + NaBr + H₂O. What type of reaction is this?

Aaddition
Bsubstitution
Celimination
Dhydrolysis
NoteC: H and Br are removed from two adjacent C atoms, forming a C=C π bond.
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
4

Hybridisation

Picture it: the carbon in CH₄

  1. Promote: one 2s electron moves up into the empty 2p orbital → four unpaired electrons
  2. Mix: the 2s and the three 2p orbitals combine → four sp³ hybrid orbitals, all at one energy, between 2s and 2p
  3. They point to the corners of a tetrahedron, 109.5° apart → each overlaps an H 1s orbital head-on: four identical C–H σ bonds
4

Hybridisation

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, and the large lobe is used in bonding. Hybrid orbitals form σ bonds, unhybridised p orbitals form π bonds.

4

Hybridisation

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):

RegionsHybridisationShape
4 (all single)sp³tetrahedral
3 (one C=C or C=O)sp²trigonal planar
2 (C≡C, or two C=C)splinear

Single bond: 1σ. Double: 1σ + 1π. Triple: 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.

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

Hybridisation

Your turnFor propene, CH₃CH=CH₂, state the hybridisation of each C, and the number of σ and π bonds.

CH₃: sp³, CH and CH₂: sp²[1]
6 C–H + 2 C–C = 8 σ, 1 π[1]

Quick checkWhat is the hybridisation of the C in CO₂?

Asp³
Bsp²
Csp
Dnone
NoteC: two C=O bonds, so 2 regions of electron density: linear, sp.
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 2py and of the 2pz orbitals, at right angles to each other
5

Shapes and Bonding in Organic Molecules

  • 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

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

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

  1. sp² orbital has more s character than sp³
  2. → smaller orbital, electrons closer to the nucleus
  3. → more effective overlap, shorter and stronger bond
6

Reagents and Bond Fission

6.1Types of reagent

ReagentDefinitionExamplesAttacks
Nucleophile
(Lewis base)		an electron pair donor, with a lone pair of electronsOH⁻, CN⁻, Cl⁻,
NH₃, H₂O, ROH		an electron-poor centre, e.g. a Cδ+ atom bonded to a halogen
Electrophile
(Lewis acid)		an electron pair acceptor, electron deficientH⁺, NO₂⁺,
Brδ+–Brδ−, Hδ+–Cl		an electron-rich centre, e.g. C=C or a benzene ring
Free radicala species with an unpaired electronCl•, Br•, •CH₃C–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.
6

Reagents and Bond Fission

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

Reagents and Bond Fission

6.4Drawing curly arrows

  • An arrow starts at a bond or a lone pair and points to where the electrons go
  • Full arrowhead: a pair of electrons moves
  • Half (fish-hook) arrowhead: one electron moves
  • Electrons flow from electron-rich to electron-poor sites
  • Show δ+ / δ−, lone pairs and charges on the species involved
7

Naming a Mechanism

Answer structuremechanism name = attacking species + type of reaction

  1. Look at the first step: what attacks the organic molecule? (nucleophile, electrophile or free radical)
  2. Compare the starting reactant with the final product (ignore intermediates): is a group replaced (substitution) or are two molecules combined (addition)?
ReactionAttacked byMechanism
CH₂=CH₂ + Br₂ → CH₂BrCH₂BrBrδ+ attacks electron-rich C=Celectrophilic addition
CH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻OH⁻ attacks Cδ+nucleophilic substitution
CH₄ + Cl₂ → CH₃Cl + HCl (UV)Cl• attacks C–Hfree radical substitution
C₆H₆ + Br₂ → C₆H₅Br + HBr (FeBr₃)Br⁺ attacks the benzene ringelectrophilic substitution
8

Factors Affecting Reactivity

8.1Inductive effect

  • In a polar bond, the more electronegative atom pulls the bonding electrons towards itself, which shifts electrons in the neighbouring bonds too
  • Electron-donating (+I): alkyl groups, increasing with size: –CH₃ < –C₂H₅ < –CH(CH₃)₂ < –C(CH₃)₃
  • Electron-withdrawing (–I): 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

Factors Affecting Reactivity

8.2Stability of carbocations

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

  1. Alkyl groups are electron-donating
  2. → three alkyl groups disperse the positive charge on C⁺ to a greater extent than one
8

Factors Affecting Reactivity

8.3Stability of anions and acid strength

Arrange in increasing Ka:

ClCH₂CH₂CH₂CO₂H < CH₃CHClCH₂CO₂H < CH₃CH₂CHClCO₂H

  1. –Cl is electron-withdrawing: it disperses the negative charge on the carboxylate ion
  2. The closer Cl is to –CO₂⁻, the stronger the effect → more stable anion
  3. → the acid dissociates more readily → larger Ka
8

Factors Affecting Reactivity

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. OH⁻ must attack the C of C–Br from the side opposite Br: easy in CH₃Br, blocked in (C₆H₅)₃CBr.

8

Factors Affecting Reactivity

Quick checkWhich carbocation is the most stable?

ACH₃⁺
BCH₃CH₂⁺
C(CH₃)₂CH⁺
D(CH₃)₃C⁺
NoteD: three electron-donating alkyl groups disperse the positive charge the most.
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

Isomerism

9.1Constitutional (structural) isomerism

TypeThe isomers differ inExample
Chainthe carbon skeleton (branching)
butane2-methylpropane
Positionthe position of the same functional group
propan-1-olpropan-2-ol
Functional groupthe functional group, e.g. an aldehyde and a ketone
propanalpropanone
9

Isomerism

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.
9

Isomerism

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
9

Isomerism

9.4Properties of enantiomers

PropertyEnantiomers are
Physicalidentical, except the direction in which they rotate plane-polarised light (equal extent, opposite directions)
Chemicalidentical, except in their interactions with another chiral molecule, e.g. an enzyme
Biologicaloften 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

Isomerism

9.5Deducing isomers

  1. Constitutional isomers: vary the chain, the position of the group and the functional group
  2. For each, look for a C=C with two different groups on each C: cis–trans isomers
  3. Look for carbons with four different groups: chiral centres

Number of stereoisomers = 2n, where n = number of chiral centres + number of C=C bonds that show cis–trans isomerism.

Exam tipIn a skeletal formula, ignore the bends and the ends. Chiral centres are only at the intersections.

Your turnHow many stereoisomers does pent-3-en-2-ol, CH₃CH=CHCH(OH)CH₃, have?

It has one chiral centre and one C=C bond that shows cis–trans isomerism.[1]
Number of stereoisomers = 2² = 4[1]

Exit ticket

1Define a nucleophile.

An electron pair donor, with a lone pair of electrons.[1]

2State the hybridisation and bond angle of C in ethene.

sp², 120°.[1]

3What is homolytic fission?

The bonding electrons are shared equally, one to each atom, forming two free radicals.[1]

4Name the mechanism: CH₃Br + CN⁻ → CH₃CN + Br⁻.

Nucleophilic substitution.[1]