Organic Chemistry Essentials

Grade 12 · Science · Study Guide

A comprehensive review of carbon-based compounds, functional groups, and reaction mechanisms central to organic chemistry.

Organic chemistry is the branch of chemistry focused on carbon-containing compounds. Carbon's ability to form four covalent bonds and chain with itself makes it uniquely suited to building the complex molecules of life.

Hydrocarbons form the backbone of organic chemistry. Alkanes contain only single C-C bonds (saturated), alkenes contain at least one C=C double bond, and alkynes contain a C≡C triple bond. Aromatic compounds like benzene have delocalized pi electrons that grant exceptional stability.

Functional groups determine the chemical behavior of organic molecules. Hydroxyl groups (-OH) characterize alcohols; carbonyl groups (C=O) appear in aldehydes and ketones; carboxyl groups (-COOH) define carboxylic acids; amino groups (-NH2) define amines. Recognizing functional groups allows prediction of reactivity.

Reaction mechanisms describe how bonds break and form. In nucleophilic substitution (SN1/SN2), an electron-rich nucleophile replaces a leaving group. In addition reactions, atoms add across a double bond. Elimination reactions remove atoms to form a double bond. Understanding stereochemistry — the 3D arrangement of atoms — is critical, as enantiomers can have vastly different biological properties.

Key terms

Isomers
Molecules with the same molecular formula but different structural arrangements or spatial configurations.
Functional group
A specific atom or group of atoms within a molecule that determines its characteristic chemical reactions.
Nucleophile
An electron-rich species that donates electrons to an electrophile during a chemical reaction.
Enantiomers
Non-superimposable mirror-image molecules that rotate plane-polarized light in opposite directions.
Polymer
A large macromolecule formed by the repeated linking of smaller monomer units through covalent bonds.
Saponification
The alkaline hydrolysis of an ester to yield a carboxylate salt and an alcohol, the basis of soap-making.

Flashcards

Alkane
A saturated hydrocarbon with only single C-C bonds; general formula CnH2n+2.
Alkene
An unsaturated hydrocarbon containing one or more C=C double bonds; general formula CnH2n.
Aldehyde
An organic compound with a carbonyl group (-CHO) at the end of a carbon chain.
Ketone
An organic compound with a carbonyl group (C=O) bonded to two carbon atoms within the chain.
Ester
A compound formed from a carboxylic acid and an alcohol with loss of water; characterized by -COO- linkage.
Electrophilic addition
A reaction in which an electrophile adds across a C=C or C≡C bond, breaking the pi bond.
Chirality
The property of a molecule that makes it non-superimposable on its mirror image, like left and right hands.
Condensation reaction
A reaction in which two molecules join with the elimination of a small molecule, typically water.

Questions

  1. Which functional group is present in carboxylic acids?

    1. -OH only
    2. -COOH
    3. -NH2
    4. -CHO
    Show answer

    Answer: -COOH

    Carboxylic acids are defined by the -COOH (carboxyl) group, which combines a carbonyl and a hydroxyl group on the same carbon.

  2. Alkenes are considered unsaturated hydrocarbons because they contain at least one carbon-carbon double bond.

    Show answer

    Answer: True

    Unsaturated means not fully saturated with hydrogen; the C=C double bond uses up bonding capacity that would otherwise hold H atoms.

  3. The process by which a nucleophile attacks a carbon bearing a leaving group in a single concerted step is called _____ substitution.

    Show answer

    Answer: sn2

    SN2 (bimolecular nucleophilic substitution) proceeds via a single transition state where the nucleophile attacks as the leaving group departs simultaneously.

  4. Benzene's unusual stability is best explained by:

    1. Three isolated double bonds
    2. Delocalized pi electrons across the ring
    3. A high molecular weight
    4. Presence of nitrogen atoms
    Show answer

    Answer: Delocalized pi electrons across the ring

    Benzene's six pi electrons are delocalized over all six carbons in the ring, a resonance stabilization that makes benzene far more stable than a simple triene.

  5. Which of the following pairs are enantiomers?

    1. Structural isomers with different connectivity
    2. Mirror-image molecules that cannot be superimposed
    3. Molecules with the same 3D shape
    4. Constitutional isomers with the same functional group
    Show answer

    Answer: Mirror-image molecules that cannot be superimposed

    Enantiomers are non-superimposable mirror images — they have the same connectivity but opposite spatial arrangements at one or more chiral centers.

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