Chemistry (9701) | An introduction to A Level organic chemistry

An Introduction to A Level Organic Chemistry (9701)

Welcome to the incredible world of Organic Chemistry! This is where you study compounds based on carbon—literally the chemistry of life. If you found the AS introductory material a little challenging, don't worry! We will break down the essential concepts of structure, naming, and reaction types step-by-step. Mastering this foundation is crucial because all A Level organic chemistry builds directly upon these ideas. Let's get started!

Quick Review: The Language of Carbon

The unique nature of carbon, able to form four strong covalent bonds and link together endlessly, is the reason organic chemistry exists.

Key Definition:

A Hydrocarbon is a compound made up of carbon (\(C\)) and hydrogen (\(H\)) atoms only.

Did you know? Alkanes are the simplest hydrocarbons and contain no functional groups—they are the 'plain vanilla' of organic molecules.

1. Formulas and Functional Groups (Syllabus 13.1)

Organic molecules are represented in several ways. You must be able to switch between these formats seamlessly.

1. General Formula: Shows the ratio of atoms for a whole homologous series (e.g., Alkanes: \(\text{C}_n\text{H}_{2n+2}\)).
2. Molecular Formula: Shows the actual number of atoms in one molecule (e.g., Ethane: \(\text{C}_2\text{H}_6\)).
3. Empirical Formula: Shows the simplest whole-number ratio of atoms (e.g., Ethene \(\text{C}_2\text{H}_4\) has an empirical formula of \(\text{CH}_2\)).
4. Structural Formula: Shows the minimum detail needed to differentiate isomers (e.g., \(\text{CH}_3\text{CH}_2\text{OH}\) for ethanol).
5. Displayed Formula: Shows all atoms and all bonds (like a full stick diagram).
6. Skeletal Formula: The shortest, most common way A Level chemists draw molecules. Carbon atoms are implied at the ends and corners of lines. Hydrogen atoms attached to carbons are also usually omitted.

Functional Groups: The Deciding Factor

A functional group is a specific group of atoms within a molecule that dictates its characteristic physical and chemical properties. Organic compounds with the same functional group belong to the same homologous series (e.g., all alcohols have the \(-OH\) group).

Key Functional Groups to Recognise (AS/A Level):

• Alkene: \(\text{C}=\text{C}\) double bond.
• Halogenoalkane: \(-X\) (where \(X = \text{Cl}, \text{Br}, \text{I}\)).
• Alcohol/Hydroxyl: \(-OH\).
• Aldehyde/Ketone (Carbonyl): \(\text{C}=\text{O}\) group. (Aldehyde: \(\text{C}=\text{O}\) at end; Ketone: \(\text{C}=\text{O}\) in middle).
• Carboxylic Acid (Carboxyl): \(-COOH\).
• Ester: \(-COO-\) linkage.
• Amine (Primary): \(-NH_2\).
• Nitrile: \(-C\equiv N\).
• Acyl Chloride: \(-COCl\) (Carboxylic acid derivative). (A Level)
• Arene: Benzene ring. (A Level)

Naming (Nomenclature) Checklist

Systematic naming (IUPAC) follows simple rules:

1. Find the longest carbon chain containing the functional group.
2. Number the chain to give the functional group the lowest possible number.
3. Identify the prefix based on chain length (1=meth, 2=eth, 3=prop, 4=but, 5=pent, 6=hex).
4. Add the suffix for the functional group (e.g., -ol for alcohol, -al for aldehyde, -one for ketone).
5. Name and number any substituents (branches).

Example: \(\text{CH}_3\text{CH}_2\text{OH}\) is Ethanol (2 carbons, suffix -ol).

2. The Mechanisms of Organic Reactions (Syllabus 13.2)

In A Level chemistry, we don't just ask what happens, we ask how it happens—this is the study of mechanisms.

2.1 Saturated vs. Unsaturated

• Saturated: Only single bonds (e.g., alkanes). These typically undergo substitution reactions.
• Unsaturated: Contains at least one double or triple bond (e.g., alkenes, alkynes). These typically undergo addition reactions across the multiple bond.

2.2 Bond Fission: Breaking the Bonds

There are two ways a covalent bond can break:

1. Homolytic Fission: The bond splits evenly, with one electron going to each atom.

   \(A-B \rightarrow A\bullet + B\bullet\)

   This forms free radicals (species with one or more unpaired electrons), often seen in reactions involving UV light or high heat.

2. Heterolytic Fission: The bond splits unevenly, with both electrons going to one atom.

   \(A-B \rightarrow A^+ + B^-\)

   This forms charged ions (cations and anions), typical in reactions involving polar molecules.

2.3 Attacking Species: Nucleophiles and Electrophiles

These are the core attacking particles that initiate reactions:

• Nucleophile (Nu): A species (atom, molecule, or ion) that is electron-rich and wants to donate a pair of electrons to form a new covalent bond. They are attracted to positive centres (nuclei).

  Analogy: A Nucleophile is a 'Love-seeker' (it seeks positive nuclei).
  Examples: \(\text{OH}^-\), \(\text{NH}_3\), \(\text{CN}^-\), water (\(\text{H}_2\text{O}\)).

• Electrophile (E+): A species that is electron-poor (often positively charged) and wants to accept a pair of electrons. They are attracted to electron-rich areas (like a \(\text{C}=\text{C}\) double bond).

  Analogy: An Electrophile is a 'Help-seeker' (it needs electrons).
  Examples: \(\text{H}^+\), \(\text{NO}_2^+\), \(\text{Br}^+\).

Mechanism Summary: The Four Pillars

1. Free-Radical Substitution (F-R.S): Uses free radicals. Common for alkanes (e.g., halogenation of ethane). Involves three steps: Initiation, Propagation, Termination.
2. Electrophilic Addition (E.A.): Attacking species is an electrophile. Common for alkenes (reacting with \(\text{Br}_2\)).
3. Nucleophilic Substitution (\(\text{S}_{\text{N}}\)): Attacking species is a nucleophile, replacing another group. Common for halogenoalkanes (e.g., with aqueous \(\text{NaOH}\)).
4. Nucleophilic Addition (N.A.): Attacking species is a nucleophile, adding across a double bond. Common for carbonyl compounds (\(\text{C}=\text{O}\)).
5. Electrophilic Substitution (E.S.): Attacking species is an electrophile, replacing a hydrogen atom. Crucial mechanism for Arenes (Benzene). (A Level)
6. Addition-Elimination (A-E): A two-step process where an addition occurs, followed immediately by elimination (e.g., acyl chloride reactions). (A Level)

IMPORTANT: Curly Arrows
In mechanisms, you must use curly arrows (\(\curvearrowright\)) to represent the movement of a pair of electrons. The arrow must start at an electron source (a bond or a lone pair) and end at the electron sink (the atom accepting the pair).

3. Shapes, Hybridization, and Bonds (Syllabus 13.3 & 29.3)

The shapes of organic molecules are determined by the hybridisation state of the carbon atoms, linking back directly to your bonding concepts (Topic 3.4).

3.1 Sigma (\(\sigma\)) and Pi (\(\pi\)) Bonds

• Sigma (\(\sigma\)) Bond: Formed by the direct (head-on) overlap of orbitals (s-s, s-p, or hybrid-hybrid). This is the strongest type of covalent bond and allows for free rotation around the bond axis. All single bonds are \(\sigma\) bonds.
• Pi (\(\pi\)) Bond: Formed by the sideways overlap of two adjacent p-orbitals, one above and one below the \(\sigma\) bond axis. This bond is weaker than the \(\sigma\) bond and restricts rotation.
• Double Bond: Consists of one \(\sigma\) bond and one \(\pi\) bond.
• Triple Bond: Consists of one \(\sigma\) bond and two \(\pi\) bonds.

3.2 Hybridization and Shape

Carbon achieves different geometries by mixing its atomic orbitals (\(2s\) and \(2p\)) to form hybrid orbitals:

3.3 Benzene: The Delocalised \(\pi\) System (A Level)

Benzene (\(\text{C}_6\text{H}_6\)) is a flat, cyclic molecule where every carbon atom is \(\mathbf{sp^2}\) hybridised.

• Each carbon forms three \(\sigma\) bonds (\(120^\circ\) bond angles).
• Each carbon has one remaining unhybridised p-orbital, sticking out above and below the ring.
• These six p-orbitals overlap sideways to form a continuous ring of electron density—the delocalised \(\pi\) system.

This delocalisation provides extreme stability (aromatic stabilisation) to the ring, explaining why benzene prefers to undergo Electrophilic Substitution rather than the Electrophilic Addition typical of regular alkenes.

4. Isomerism: Molecules with the Same Formula (Syllabus 13.4 & 29.4)

Isomers are compounds that have the same molecular formula but different arrangements of atoms.

4.1 Structural Isomerism (AS Level)

Isomers that have the same molecular formula but different structural formulas (different bonding connections).

1. Chain Isomerism: Different arrangements of the carbon skeleton (straight chain vs. branched chain).
   Example: Butane (\(\text{C}_4\text{H}_{10}\)) vs. 2-methylpropane.
2. Positional Isomerism: Same carbon skeleton, but the functional group or substituent is placed at a different position.
   Example: Propan-1-ol (\(\text{CH}_3\text{CH}_2\text{CH}_2\text{OH}\)) vs. Propan-2-ol.
3. Functional Group Isomerism: Molecules belong to entirely different homologous series.
   Example: Ethanol (\(\text{CH}_3\text{CH}_2\text{OH}\)) and Dimethyl ether (\(\text{CH}_3\text{OCH}_3\)) are functional group isomers of \(\text{C}_2\text{H}_6\text{O}\).

4.2 Stereoisomerism (Geometrical and Optical)

Isomers that have the same structural formula but the atoms are arranged differently in 3D space.

A) Geometrical Isomerism (cis/trans)

This occurs in alkenes because the presence of the \(\pi\) bond restricts rotation around the \(\text{C}=\text{C}\) axis.

Condition: Each carbon atom of the double bond must be bonded to two different groups.

• cis: Identical or high-priority groups are on the same side of the double bond.
• trans: Identical or high-priority groups are on opposite sides of the double bond.

Example: But-2-ene can exist as cis-but-2-ene and trans-but-2-ene.

B) Optical Isomerism (Enantiomers) (AS/A Level)

Optical isomerism arises when a molecule is non-superimposable on its mirror image.

Chiral Centre: A carbon atom bonded to four different groups. This is also called an asymmetric carbon atom.

The two mirror image forms are called enantiomers.

Optically Active: A substance is optically active if it can rotate the plane of plane-polarised light. One enantiomer rotates the light clockwise (+), and the other rotates it anticlockwise (-).

Racemic Mixture: A mixture containing equal amounts of both enantiomers. Because the rotations cancel out, a racemic mixture is optically inactive.

5. Review: Key Takeaways

To succeed in organic chemistry, you must treat structure, naming, and reactions as integrated topics.

• Functional groups are the heart of reactivity. Learn to identify them instantly using structural or skeletal formulas.
• Mechanisms are the reason for reactions. Understand the roles of electron-rich Nucleophiles (Nu) and electron-poor Electrophiles (E+).
• Bonding dictates shape: \(\text{sp}^3\) (tetrahedral, free rotation), \(\text{sp}^2\) (trigonal planar, restricted rotation, necessary for cis/trans).
• Isomerism tests your 3D visualisation. Look for chain position differences (structural) and the four-different-groups rule (optical).

Keep practising drawing mechanisms (using those curly arrows!) and deducing structures from names. You've got this!