Introduction to Organic Chemistry
1. What is an Organic Compound?
Organic chemistry = study of organic compounds.
Definition: Organic compounds = all compounds containing carbon EXCEPT
- oxides of carbon — CO and CO₂
- carbonates & hydrogencarbonate ions — CO₃²⁻ , HCO₃⁻
- carbonic acid, H₂CO₃
- hydrogen cyanide (HCN) and the cyanide ion CN⁻
Long ago people believed organic compounds could only be made by living things (humans, plants, insects…). Then chemists made them in the lab — so the name stayed but the idea changed.
2. Functional Groups
Definition: An atom or group of atoms that replaces hydrogen in an organic compound, defines the structure of a family and determines its chemical reactions.
Every functional group ⇒ its own “branch” = a whole set of reactions. That is why we study them one by one in AS.
| # | Family | Functional group | # | Family | Functional group |
|---|---|---|---|---|---|
| 1 | Halogenoalkane | –X (X = Cl, Br, I) | 7 | Carboxylic acid | –COOH (C(=O)–O–H) |
| 2 | Primary alcohol | R–CH₂–OH | 8 | Ester | –COO– (C(=O)–O–) |
| 3 | Secondary alcohol | R–CH(R′)–OH | 9 | Acyl chloride | –COCl (C(=O)–Cl) |
| 4 | Tertiary alcohol | R–C(R′)(R″)–OH | 10 | Amide | –CONH₂ (C(=O)–NH₂) |
| 5 | Aldehyde | –CHO (C(=O)–H) | 11 | Nitrile | –C≡N |
| 6 | Ketone | –C(=O)– (C=O between two C) |
3. Homologous Series
Definition: A family of organic compounds with the same general formula; successive members differ by CH₂. They have similar chemical properties.
Alkenes CnH2n
Alcohols CnH2n+1OH
Carboxylic acids CnH2n+1COOH
4. Naming Organic Compounds
① Prefixes & alkyl groups
| C | Alkane | Formula | Alkyl | Formula |
|---|---|---|---|---|
| 1 | methane | CH₄ | methyl | CH₃ |
| 2 | ethane | C₂H₆ | ethyl | C₂H₅ |
| 3 | propane | C₃H₈ | propyl | C₃H₇ |
| 4 | butane | C₄H₁₀ | butyl | C₄H₉ |
| 5 | pentane | C₅H₁₂ | pentyl | C₅H₁₁ |
| 6 | hexane | C₆H₁₄ | hexyl | C₆H₁₃ |
| 7 | heptane | C₇H₁₆ | heptyl | C₇H₁₅ |
| 8 | octane | C₈H₁₈ | octyl | C₈H₁₇ |
| 9 | nonane | C₉H₂₀ | nonyl | C₉H₁₉ |
| 10 | decane | C₁₀H₂₂ | decyl | C₁₀H₂₁ |
Alkane = only C and H, every C has 4 single bonds ⇒ saturated. Alkyl group = alkane minus one H (methane → methyl, ethane → ethyl …).
Memory: meth (1) · eth (2) · prop (3) · but (4) · pent (5) · hex (6) · hept (7) · oct (8) · non (9) · dec (10)
② The 5 naming rules
- Choose the longest chain (pick the straightest one — easier to see).
- Number from the end where a branch / functional group gets the lowest number.
- Numbers separated by commas (2,2,3); numbers and words separated by hyphens (3-ethyl-2-methylpentane).
- Same group repeated ⇒ di, tri …
- Different groups ⇒ quote in alphabetical order (ethyl before methyl — e before m; “di/tri” are ignored for ordering).
③ Practice from the board (all alkanes — every C has 4 bonds)
| # | Structure (condensed) | Thinking | Name |
|---|---|---|---|
| 1 | CH₃CH₂CH₂CH(CH₃)CH₃ | longest = 5 C; number from the CH₃ end so the branch gets 2 (not 4) | 2-methylpentane |
| 2 | CH₃C(CH₃)₂CH(CH₃)CH₂CH₃ | 5 C chain; number from left: branches on 2, 2, 3 ⇒ all methyl ⇒ tri | 2,2,3-trimethylpentane |
| 3 | CH₃CH(CH₃)CH₂CH₃ | the “CH₂CH₃” arm is part of the chain: 4 C, not a 2-C branch | 2-methylbutane |
| 4 | CH₃CH(CH₃)CH(C₂H₅)CH₂CH₃ | 5 C chain; methyl on 2, ethyl on 3 ⇒ ethyl first (alphabet) | 3-ethyl-2-methylpentane |
| 6 | CH₃CH₂C(CH₃)(C₂H₅)CH₂CH₂CH₂CH₃ | 7 C chain; both groups on C3 ⇒ 3-ethyl, 3-methyl | 3-ethyl-3-methylheptane |
| 7 | CH₃CH₂CH(C₂H₅)CH₂CH(C₂H₅)CH₂CH₃ | 7 C chain; both branches ethyl ⇒ “di”, numbers 3,5 (either end gives same) | 3,5-diethylheptane |
| 8 | CH₃CH(CH₃)CH₂CH(CH₃)CH₂CH(CH₃)CH₂CH₃ | 8 C chain; methyls on 2, 4, 6 | 2,4,6-trimethyloctane |
| 9 | CH₃CH(C₂H₅)CH₂CH(C₂H₅)CH₂CH(C₂H₅)CH₂CH₃ | 8 C chain; ethyls on 2, 4, 6 | 2,4,6-triethyloctane |
5. Types of Organic Formula
| Formula | What it shows | e.g. ethanol |
|---|---|---|
| Molecular | just the atoms & their numbers — no structure information | C₂H₆O |
| Structural | the order in which atoms are bonded to each other | CH₃CH₂OH |
| Displayed | ALL atoms and ALL bonds drawn out | H–C(H)(H)–C(H)(H)–O–H |
| Skeletal | carbon chain as a zig-zag; C and H (on C) removed; functional groups shown | zig-zag with OH |
Structural formula is the most useful: it lets you see isomerism — e.g. but-1-ene CH₂=CHCH₂CH₃ and but-2-ene CH₃CH=CHCH₃ have the same molecular formula but different structure.
Skeletal conventions: each line-end / corner is a carbon; hydrogens on carbon not drawn; heteroatoms (O, Cl, Br, N) and –OH written out; double bond = double line.
Board practice — draw structural, displayed and skeletal formula for:
| # | Name | Structural formula |
|---|---|---|
| 1 | 2-methylbutane | CH₃CH(CH₃)CH₂CH₃ |
| 2 | 3,5-diethylheptane | CH₃CH₂CH(C₂H₅)CH₂CH(C₂H₅)CH₂CH₃ |
| 3 | 2,4,6-trimethyloctane | CH₃CH(CH₃)CH₂CH(CH₃)CH₂CH(CH₃)CH₂CH₃ |
| 4 | 3-ethyl-3-methylheptane | CH₃CH₂C(CH₃)(C₂H₅)CH₂CH₂CH₂CH₃ |
| 5 | but-1-ene | CH₂=CHCH₂CH₃ |
| 6 | but-2-ene | CH₃CH=CHCH₃ |
| 7 | propan-1-ol | CH₃CH₂CH₂OH |
| 8 | propan-2-ol | CH₃CH(OH)CH₃ |
| 9 | 2,2,4-trimethylpentane | CH₃C(CH₃)₂CH₂CH(CH₃)CH₃ |
Skeletal of propan-2-ol: zig-zag of 3 C with OH on the middle carbon; 2,2,4-trimethylpentane: 5-C chain with two methyl spikes on C2 and one on C4 (see figure).
6. Isomerism
Definition: Isomers = compounds with the same molecular formula but different arrangement of atoms.
Structural isomerism — three types
| Type | Cause | Board example |
|---|---|---|
| Chain | branching of the carbon chain (alkyl groups) | butane CH₃CH₂CH₂CH₃ vs 2-methylpropane CH₃CH(CH₃)CH₃ |
| Position | position of the functional group changes | 1-bromopropane vs 2-bromopropane |
| Functional group | a different functional group altogether | propanal CH₃CH₂CHO vs propanone CH₃COCH₃ |
Q. Draw the isomers of butanol (C₄H₉OH)
Method: keep 4 C chain and move OH (position) ⇒ then branch the chain (chain isomerism) ⇒ then move OH again.
OH group must get the lowest number — numbering starts from the OH end. “Butanol is an important compound — learn its 4 isomers; it is the usual examiner’s favourite (but-…).”
Fast method for counting isomers in P1 (C₃H₅Cl₃ = trichloropropane)
Don’t draw H. Draw the carbon skeleton C–C–C and place the Cl’s. Start with all three on one carbon, then move one by one, then two by two.
Answer: C
1 + 2 + 2 + 2 = 7
Answer: C (7)
Skeletal formula MCQs (board)
| Q | Question | Working / answer |
|---|---|---|
| 30 | skeletal of 2-chloropentan-3-ol | 5-C chain; Cl on C2, OH on C3 ⇒ A ✓; C, D have wrong chain length/positions |
| 34 | skeletal of 2-methylpentan-1-ol | OH on C1 end, methyl on C2 ⇒ C ✓ |
| 38 | skeletal of 2-bromopentan-4-ol | (properly 4-bromopentan-2-ol: OH gets lowest number) 5-C chain, Br and OH on C2/C4 ⇒ A ✓ |
| 40 | three structural isomers of C₅H₁₂ — which formulae correctly represent them? | pentane CH₃CH₂CH₂CH₂CH₃; 2-methylbutane CH₃CH(CH₃)CH₂CH₃ (= CH₃CH(CH₃)CH₂CH₃); 2,2-dimethylpropane C(CH₃)₄ = CH₃C(CH₃)₂CH₃ ⇒ C ✓ |
| 53 | jet fuel Q = C₁₄H₃₀; which skeletal is a structural isomer of Q? | need same molecular formula but different structure: B is C₁₃H₂₈ ✗; A and D crossed on board; C ✓ (C₁₄H₃₀) |
7. Stereoisomerism
Definition: Same structural formula, but atoms arranged differently in 3-D space. Two kinds: optical and geometric (cis–trans).
A. Optical isomerism
- Arises when a carbon is bonded to 4 different atoms/groups ⇒ a chiral centre (mark with *).
- The two isomers are non-superimposable mirror images of each other.
- They rotate plane-polarised light in opposite directions.
Sir’s fast method to count chiral centres
Cross out CH₃, CH₂ and C=O carbons and the C with 2 CH₃ ⇒ only the two bridgehead carbons remain; each has 4 different chains (one arm leads to C=O, the other to CH₂–CH₂).
Answer: C (2)
Cross out all ring carbons (benzene, lactone ring), C=O, CH₃, CH₂ ⇒ only one CH left (bonded to H, phenyl ring, ring carbon, CH₂COCH₃) — a chiral centre. “2 minutes and done.”
Answer: B (1)
A, B, C each contain a carbon with 4 different groups. In D the central C carries two identical hydroxyphenyl groups ⇒ not chiral.
Answer: D
Cross out C=C, C=O, CH₂ ⇒ two CH carbons left (ring C–H(–O) and side-chain CH(OH)), each with 4 different groups.
Answer: B (2)
Same method: CH₃, CH₂ and C=O carbons crossed out; the remaining bridgehead/ring carbons bonded to 4 different groups are marked *. Board count = 7.
Answer: C (7)
B. Geometric (cis–trans) isomerism
- Arises because the C=C π bond prevents rotation about the double bond.
- Two different groups must be attached to EACH carbon of the C=C.
- cis = the two chosen groups on the same side; trans = on opposite sides.
- A molecule with n such C=C bonds (different groups on each carbon) has 2ⁿ geometric isomers.
Past-paper questions — stereo & counting
| Q | Question | Working / answer |
|---|---|---|
| 3 | structural + cis-trans isomers of dichloropropene C₃H₄Cl₂ | ① CH₂=CH–CHCl₂ ✗ ② CH₂=CCl–CH₂Cl ✗ ③ CHCl=CH–CH₂Cl ✓ (cis+trans) ④ CHCl=CCl–CH₃ ✓ (cis+trans) ⑤ CCl₂=CH–CH₃ ✗ ⇒ 1+1+2+2+1 = 7 ⇒ D |
| 5 | alcohols C₄H₁₀O incl. stereoisomers | butan-1-ol, butan-2-ol (has chiral C* ⇒ 2 optical isomers), 2-methylpropan-1-ol, 2-methylpropan-2-ol ⇒ 1+2+1+1 = 5 ⇒ C |
| 11 | which alkene has cis and trans forms | need two different groups on each C=C carbon: CH₃CH₂CH₂CH=CHCH₃ ⇒ B ✓; A (=CH₂), C (=CH₂ end) and D (CH₃/CH₃ on one carbon) ✗ |
| 26 | CH₃CH=CRCH₂CH₃ has no cis-trans. What is R? | The right carbon already has CH₂CH₃; if R = C₂H₅ both groups on that carbon are identical ⇒ no cis-trans ⇒ D (C₂H₅) |
| 32 | compound showing both cis-trans and optical isomerism | needs a C=C with two different groups on each C and a chiral C — board circles B (A and D ✗; C is also ticked on the board — check against the mark scheme) |
| 39 | isomers of C₂H₂Br₂ incl. stereo | ① CH₂=CBr₂ ② CHBr=CHBr → cis + trans ⇒ 1 + 2 = 3 ⇒ B |
8. Counting σ and π bonds
Every single bond = 1σ · every double bond = 1σ + 1π · every triple = 1σ + 2π. C–H bonds are σ too — count them all!
CH₂=CH–CH₂–C(=O)–O–CH₂–CH=CH₂. Board labels each bond σ / π: the 3 C=C/C=O double bonds contribute 3π (and 3σ) ⇒ total σ = 18.
Answer: C (18)
- Organic = carbon compounds except CO, CO₂, carbonates/HCO₃⁻, H₂CO₃, HCN/CN⁻.
- Functional group = atom/group replacing H that decides reactions; homologous series differ by CH₂.
- Naming: longest chain → lowest numbers → commas between numbers, hyphens number–word → di/tri → alphabetical (ethyl before methyl).
- Formulae: molecular (atoms) · structural (bonding order) · displayed (all bonds) · skeletal (zig-zag; show functional groups).
- Isomers: same molecular formula. Structural = chain / position / functional group. Stereo = optical + geometric.
- Optical: chiral C* with 4 different groups; non-superimposable mirror images; rotate plane-polarised light oppositely. CH₃, CH₂, C=C carbons, rings can’t be chiral.
- Geometric: C=C (no rotation) + two different groups on each carbon; cis = same side, trans = opposite; n C=C ⇒ 2ⁿ.
- Counting isomers fast: no H, group all substituents first, move one-by-one; position first, then chain.