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Organic Chemistry Fundamentals

• Organic Chemistry is the study of compounds containing Carbon ($C$) and Hydrogen ($H$), often including Nitrogen ($N$) and Oxygen ($O$).

• Exceptions: Carbonates ($CO_3^{2-}$) and oxides of carbon ($CO_2$, $CO$) are generally considered inorganic.

• Taranaki is New Zealand’s energy province, generating $NZ\,2.5\,\text{billion}$ for the GDP and accounting for $90\,\%$ of the industry's nationwide employment.

Structure and Formulae

• Bonding Requirements: Carbon ($C$) needs $4$ bonds; Hydrogen ($H$) needs $1$; Oxygen ($O$) needs $2$; Halogens ($X$) need $1$.

• Graphic (Structural) Formulae: Show all atoms and all bonds clearly.

• Condensed Structural Formulae: Show atom positions without showing every individual bond (e.g., $CH_3CH_2CH_2CH_2CH_3$ for pentane).

• Conformers: Different drawings of the same molecule resulting from rotation around single bonds.

Isomerism

• Structural Isomers: Compounds with the same molecular formula but different structural arrangements (e.g., butane and methylpropane both share $C_4H_{10}$).

• Geometrical Isomers (Cis-Trans): Occur in alkenes due to the double bond preventing rotation. Requires each carbon in the $C=C$ bond to have two different groups attached.

  • Cis: Similar groups on the same side of the double bond.

  • Trans: Similar groups on opposite sides of the double bond.

Hydrocarbons: Alkanes, Alkenes, and Alkynes

• Alkanes: Saturated hydrocarbons with the general formula $C_n H_{2n+2}$. They are generally unreactive.

• Alkenes: Unsaturated hydrocarbons containing a $C=C$ double bond ($C_n H_{2n}$).

• Alkynes: Unsaturated hydrocarbons containing a $C\equiv C$ triple bond ($C_n H_{2n-2}$).

• Comparison Tests:

  • Bromine Water ($Br_2$): Alkanes react slowly in UV light (substitution, orange to colourless). Alkenes react immediately (addition, orange to colourless).

  • Permanganate ($KMnO_4$): Alkanes show no change. Alkenes react immediately, turning purple $KMnO_4$ into a brown precipitate ($MnO_2$).

• Ethyne Preparation: Reaction of Calcium carbide and water: $CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2$.

Chemical Reactivity Rules

• Markovnikov’s Rule: In addition reactions with unsymmetrical reagents, the Hydrogen atom adds to the carbon of the double bond that already has the most Hydrogens (‘Rich get richer’).

• Zaitsev’s Rule: In elimination reactions, the Hydrogen atom is removed from the carbon atom with the fewest Hydrogens (‘Poor get poorer’).

• Cracking: Large alkanes are split into smaller alkanes and alkenes using heat and a catalyst.

Functional Group Derivatives

• Haloalkanes ($C_n H_{2n+1}X$): Classified as primary ($1^o$), secondary ($2^o$), or tertiary ($3^o$). Reactions include substitution to form alcohols ($KOH_{(aq)}$) or elimination to form alkenes ($KOH_{(alc)}$).

• Alcohols ($C_n H_{2n+1}OH$): Classified similarly to haloalkanes. Primary and secondary alcohols can be oxidised to carboxylic acids or ketones using acidified $KMnO_4$ or $K_2Cr_2O_7$. Dehydration using conc. $H_2SO_4$ forms alkenes.

• Amines ($C_n H_{2n+1}NH_2$): Derivatives of ammonia ($NH_3$) with a fishy smell; these act as weak bases.

• Carboxylic Acids ($C_n H_{2n+1}COOH$): Weak electrolytes that react with metals, bases, and carbonates.

• Esters: Formed via esterification ($Acid + Alcohol \rightleftharpoons Ester + Water$) and typically have fruity odors.

Industrial and Historical Context

• Polymers: Formed from alkene monomers under high pressure/temperature (e.g., polyethene, PVC, polystyrene).

• Alan McDiarmid: New Zealand Nobel laureate (2000) for the discovery of conducting polymers (especially those from ethyne).

• Ahmed Zewail: Nobel laureate (1999) for femtosecond ($10^{-15}\,s$) spectroscopy of transition states.

Questions & Discussion

• Q: What does collecting ethyne gas over water tell you about it?

• A: It indicates that the gas has very low solubility in water.

• Q: How can you prove a gas is ethyne?

• A: Add bromine water; it will decolourise (orange/brown to colourless) immediately through an addition reaction.

• Q: If you performed substitution on 2-bromopentane, what conditions are needed and what is the single product?

• A: Aqueous $KOH$ (or $NaOH$) is required, and the single product is pentan-2-ol.

• Q: What happens if dilute ammonia instead of concentrated/alcoholic ammonia is used in reaction with a haloalkane?

• A: Dilute conditions may favour substitution with water or lead to poor yields compared to the heat/sealed tube method with alcoholic ammonia.