14.1 Organic Chemistry

14.1 Organic Chemistry on NMAT Chemistry

The Center for Educational Measurement (CEM) NMAT Chemistry subtest is a 30-item block with a recommended ~30 minutes. Official content areas include General Chemistry, Analytical Chemistry, Organic Chemistry, and Biochemistry. Organic items reward structure recognition, property prediction, and reaction-class logic more than multi-step synthesis puzzles.

This section trains the organic half of Part 2 Chemistry: hydrocarbons, functional groups, isomerism, basic reactions, and naming/recognition. Pair it with general chemistry (bonding, stoichiometry, acids/bases) and the biochemistry section that follows.

Quick frame: First identify the carbon skeleton and functional group. Then decide property (polarity, acidity, reactivity) or reaction class (addition / substitution / elimination / oxidation). Naming is secondary to recognition under time pressure.

Hydrocarbons — high-yield properties

Hydrocarbons contain only C and H. Saturation and bonding type control boiling points, reactivity, and typical reaction class.

Alkane cues: general formula for acyclic alkanes is CₙH₂ₙ₊₂. Branching lowers boiling point among isomers; longer chains raise boiling point via London forces.

Alkene/alkyne cues: addition of H₂, HX, X₂, or H₂O is classic. Markovnikov orientation (H attaches to the carbon already bearing more hydrogens) appears in intro problems with HX/H₂O addition to unsymmetrical alkenes.

Aromatic cue: the benzene ring resists simple addition under mild conditions; NMAT-level items often test that substitution preserves aromaticity while addition would destroy it.

Functional groups — recognition table

Memorize structure + ending + one chemical handle. On the exam, the stem may show a condensed formula, a skeletal sketch described in words, or a name fragment.

Recognition drill (mental flashcards):

1. Carbonyl at the end of a chain with H attached → aldehyde.
2. Carbonyl between two carbons → ketone.
3. Carbonyl next to –OH → carboxylic acid.
4. Carbonyl next to –OR → ester.
5. Carbonyl next to –NH₂ / –NHR / –NR₂ → amide.
6. –OH on saturated carbon, no carbonyl → alcohol.
7. –O– between two carbons, no H on O → ether.
8. Nitrogen with three single bonds to C/H, no carbonyl → amine.

Polarity ladder (rough): hydrocarbons < ethers < aldehydes/ketones < alcohols < carboxylic acids (for similar size). Hydrogen bonding (alcohols, acids, primary/secondary amines, amides) raises boiling points relative to non-H-bonding isomers.

Isomerism — structural, geometric, optical

Isomers share a molecular formula but differ in structure or spatial arrangement.

1. Structural (constitutional) isomers
Same formula, different connectivity. Examples: butane vs isobutane (C₄H₁₀); ethanol vs dimethyl ether (C₂H₆O). Functional-group isomers (alcohol vs ether; aldehyde vs ketone for C₃H₆O) are a favorite trap when two structures look “alike” at a glance.

2. Geometric (cis–trans / E–Z conceptual)
Requires restricted rotation, classically a C=C with two different substituents on each carbon of the double bond. Cis: similar groups same side; trans: opposite sides. Geometric isomers are diastereomers with different physical properties (bp, polarity). If either double-bond carbon has two identical groups, cis–trans is not possible.

3. Optical isomerism (conceptual)
A carbon with four different substituents is a chiral (asymmetric) center. Molecules that are non-superimposable mirror images are enantiomers. They share most physical properties in achiral environments but rotate plane-polarized light in opposite directions and can interact differently with chiral biological receptors — the biochemistry link NMAT loves at intro level. Racemic mixture: 1:1 enantiomers, optically inactive overall.

Exam filter: Count carbons and check connectivity first (structural). Look for C=C with proper substitution (geometric). Look for a carbon with four distinct groups (optical).

Basic reaction types

Keep a four-bin map. Classify first; details second.

Addition
Two reactants combine; π bond in alkene/alkyne becomes σ bonds. Examples: hydrogenation (H₂), halogenation (Br₂), hydrohalogenation (HBr), hydration (H₂O). Net effect: unsaturated → more saturated.

Substitution
One atom/group replaces another. Alkane halogenation (Cl₂/light), aromatic electrophilic substitution (conceptual), nucleophilic substitution on alkyl halides (intro SN idea). Product count of “pieces” stays similar; atoms swap.

Elimination
Atoms/groups leave adjacent carbons to form a π bond (alkene from alkyl halide or alcohol dehydration). Reverse of addition in a broad sense. Heat + acid for alcohol dehydration is a classic school-lab cue.

Oxidation of alcohols
Track how many H atoms are attached to the carbinol carbon (the C bearing –OH):

Tollens’ / Benedict’s style cues (recognition, not lab procedure): aldehydes oxidize easily; ketones generally do not under the same mild tests — useful when a stem asks which compound gives a positive “silver mirror” or reduces Cu²⁺.

Naming basics and recognition drills

Intro IUPAC workflow (enough for NMAT stems):

1. Find the longest continuous carbon chain containing the principal functional group.
2. Number to give the principal group the lowest possible number.
3. Name substituents as prefixes (methyl-, ethyl-, chloro-, etc.).
4. Use the correct suffix (-ane, -ene, -yne, -ol, -al, -one, -oic acid, etc.).

Common common-name survivors still seen in exams: acetic acid (ethanoic), acetone (propanone), formaldehyde (methanal), isopropyl alcohol (propan-2-ol).

Drill set (do these mentally before practice banks):

• C₂H₅OH vs CH₃OCH₃ → alcohol vs ether (functional isomers).
• CH₃CH=CH₂ + HBr → addition product (propyl vs isopropyl bromide depends on orientation).
• C₆H₆ with Br₂/FeBr₃-type conditions → substitution, not addition.
• (CH₃)₃COH + mild oxidant → no reaction (3° alcohol).
• 2-butene cis vs trans → geometric isomers; both are C₄H₈ alkenes.

NMAT timing note

Organic items rarely need long arithmetic. Spend seconds on group ID, then one property or one reaction class. If a stem mixes organic structure with stoichiometry (e.g., moles of Br₂ for complete addition to an alkyne), switch to quantitative tactics from the final section of this chapter.

You are ready for NMAT organic chemistry when you can: (1) classify any simple C/H/O/N structure into the eight functional groups above, (2) name the hydrocarbon family and predict addition vs substitution, (3) distinguish structural vs geometric vs optical isomer prompts, and (4) predict alcohol oxidation products from 1°/2°/3° classification.