10th Class Chemistry Chapter 8 Notes PDF 2026 – Organic Chemistry

Definition

An organic compound is a compound containing carbon, almost always bonded covalently, typically also with hydrogen and often other elements like oxygen or nitrogen. Ethylamine ( $C H_{3} - C H_{2} - N H_{2}$ ) is organic simply because it's built on a carbon chain, even though it contains nitrogen — the presence of carbon as the structural backbone is what defines the class, not the absence of other elements.

Organic compounds are classified by the type of carbon chain: open-chain (aliphatic) compounds have carbon atoms in a straight or branched chain with two free ends; closed-chain (cyclic) compounds have carbon atoms (or carbon plus other atoms) joined in a ring.

Definition

Saturated hydrocarbons contain only single carbon–carbon bonds (alkanes). Unsaturated hydrocarbons contain at least one double or triple carbon–carbon bond (alkenes, alkynes) — the double/triple bond makes them generally more chemically reactive than saturated compounds, since that extra bond can readily open up to form new bonds.

Definition

A homologous series is a family of compounds with the same general formula and functional group, each differing from the next by a $- C H_{2} -$ unit.

Five characteristics of a homologous series: (1) all members share the same general formula, (2) each successive member differs by $C H_{2}$ (mass difference of $14$ ), (3) all members have similar chemical properties (due to the shared functional group), (4) physical properties (like boiling point, melting point) change gradually and predictably along the series, (5) all members can generally be prepared by similar methods.

Remember

Members of a homologous series have similar chemical properties because chemical behaviour is mainly determined by the functional group present, which is identical across the whole series — the only thing changing between members is the length of the carbon chain, which mainly affects physical properties rather than the chemistry.

Definition

Alkenes contain the carbon–carbon double bond ( $C = C$ ) as their functional group; general formula $C_{n} H_{2 n}$ .

Alkynes contain the carbon–carbon triple bond ( $C \equiv C$ ); general formula $C_{n} H_{2 n - 2}$ .

Alcohols contain the hydroxyl group ( $- O H$ ); aldehydes contain $- C H O$ ; ketones contain $- C O -$ (between two alkyl groups); carboxylic acids contain $- C O O H$ ; esters contain $- C O O -$ .

What's common between the carboxyl, amide, and ester functional groups is that all three are built around the same core carbonyl ( $C = O$ ) unit, differing only in what's attached alongside it ( $- O H$ for carboxylic acid, $- N H_{2}$ / $- N H R$ for amide, $- O - R$ for ester) — this shared carbonyl core is also why they're chemically related and can often be converted into one another.

The difference between aldehyde, ketone, and carboxylic acid functional groups: an aldehyde's carbonyl carbon is bonded to at least one hydrogen ( $- C H O$ , so it always sits at the end of a chain); a ketone's carbonyl carbon is bonded to two carbon (alkyl) groups, never hydrogen ( $- C O -$ , so it sits within a chain, never at the end); a carboxylic acid's carbonyl carbon is additionally bonded to a hydroxyl group ( $- C O O H$ ).

Example 1 — naming a range of functional groups

Name the following compounds: (a) $C H_{3} - C H_{2} - C H_{2} O H$ , (b) $C H_{3} - C H_{2} - C H O$ , (c) $C H_{3} - C H_{2} - C H_{2} - C O O H$ , (d) $C H_{3} - C H_{2} - C O - O - C H_{3}$ .

(a) A $3$ -carbon chain with $- O H$ on the end carbon: an alcohol.
(b) A $3$ -carbon chain with $- C H O$ at the end: an aldehyde.
(c) A $4$ -carbon chain with $- C O O H$ : a carboxylic acid.
(d) A carbonyl linked through oxygen to another alkyl group: an ester (propanoyl part $+$ methyl part).

Answer: (a) Propan-1-ol, (b) Propanal, (c) Butanoic acid, (d) Methyl propanoate

Example 2 — writing ester formulae

Write the formula of the ester called ethyl methanoate.

'Methanoate' comes from methanoic acid ( $H C O O H$ ), giving the acid part $H C O O -$ .
'Ethyl' gives the alcohol-derived part, $- C_{2} H_{5}$ .

Answer: $H C O O C_{2} H_{5}$ (i.e. $H C O O - C H_{2} C H_{3}$ )

Example 3 — identifying an ester by name

Which ester is called ethyl propanoate?

'Propanoate' comes from propanoic acid ( $C H_{3} C H_{2} C O O H$ ), giving $C H_{3} C H_{2} C O O -$ .
'Ethyl' gives $- C_{2} H_{5}$ .

Answer: $C H_{3} C H_{2} C O O C_{2} H_{5}$

Example 4 — IUPAC name of a straight-chain alkane

Give the IUPAC name for $C H_{3} - C H_{2} - C H_{2} - C H_{3}$ .

Four carbon atoms, all single bonds (saturated), straight chain.

Answer: Butane

Example 5 — IUPAC name of an alkene

Choose the correct IUPAC name for $C H_{2} = C H - C H_{2} - C H_{3}$ .

Four carbon atoms with a double bond starting at carbon $1$ .
Number the chain to give the double bond the lowest possible number.

Answer: But-1-ene

Example 6 — writing a structural formula from a name

Write the structural formula of butan-2-ol.

'Butan' = $4$ -carbon chain; '-2-ol' = $- O H$ on carbon $2$ .

Answer: $C H_{3} - C H (O H) - C H_{2} - C H_{3}$

Definition

An alkyl group is formed when an alkane molecule loses one hydrogen atom, leaving a single bonding position free — e.g. methane ( $C H_{4}$ ) loses one H to give the methyl group ( $- C H_{3}$ ); ethane ( $C_{2} H_{6}$ ) gives the ethyl group ( $- C_{2} H_{5}$ ).

Definition

Structural isomerism is when two or more compounds share the same molecular formula but have different structural (structural) arrangements of their atoms — giving them different properties despite the identical formula.

Example 7 — isomers of a 5-carbon saturated hydrocarbon

How many structural isomers exist for a saturated hydrocarbon with five carbon atoms ( $C_{5} H_{12}$ )? Write their structural formulae.

Straight chain: $C H_{3} - C H_{2} - C H_{2} - C H_{2} - C H_{3}$ (n-pentane).
One branch: $C H_{3} - C H (C H_{3}) - C H_{2} - C H_{3}$ (2-methylbutane).
Two branches on the central carbon: $C H_{3} - C (C H_{3})_{2} - C H_{3}$ (2,2-dimethylpropane).

Answer: $3$ isomers: n-pentane, 2-methylbutane, 2,2-dimethylpropane

Example 8 — isomers of butene

Write the structural formulae of the isomers shown by butene ( $C_{4} H_{8}$ ).

Double bond at position $1$ : $C H_{2} = C H - C H_{2} - C H_{3}$ (but-1-ene).
Double bond at position $2$ : $C H_{3} - C H = C H - C H_{3}$ (but-2-ene).
A branched isomer: $C H_{2} = C (C H_{3}) - C H_{3}$ (2-methylprop-1-ene).

Answer: But-1-ene, but-2-ene, 2-methylprop-1-ene

Remember

2-methylpropane and n-butane share the molecular formula $C_{4} H_{10}$ but differ in structure (one branched, one straight-chain), giving them different physical properties — 2-methylpropane's more compact, branched shape gives it a lower boiling point than the straight-chain n-butane, since branched molecules pack less efficiently and have weaker inter-molecular forces between them.

Definition

Alicyclic compounds are saturated (or unsaturated) ring compounds made only of carbon atoms, behaving chemically much like their open-chain counterparts — e.g. cyclohexane, a saturated ring with general formula $C_{n} H_{2 n}$ .

Aromatic compounds are ring compounds based on the benzene ring: six carbon atoms joined in a ring by a special arrangement of alternating/delocalized bonds, giving benzene unusual stability and different chemical behaviour compared to a simple alicyclic ring like cyclohexane, even though both have six carbons in a ring.

Heterocyclic compounds are ring compounds that contain at least one atom other than carbon within the ring itself (commonly nitrogen or oxygen) — replacing the ring structure's uniformity and often giving these compounds distinct chemical properties important in biology (e.g. in some vitamins and DNA bases).

Definition

The key difference between cyclohexane and benzene: cyclohexane is fully saturated (single bonds only, reacts like an alkane), while benzene's delocalized bonding makes it far more stable than a simple ring with alternating single/double bonds would be, so it resists the addition reactions typical of alkenes and instead favours substitution reactions that preserve its stable ring.

Definition

When a hydroxyl ( $- O H$ ) group is attached directly to a benzene ring (rather than to a chain carbon), the compound belongs to the phenol family, not the alcohol family — phenols differ from alcohols in being noticeably more acidic, since the benzene ring stabilizes the negative charge left behind after the $- O H$ 's hydrogen is lost, making that hydrogen easier to release.

Quick Recap

●Organic compound: carbon-based (covalent), open-chain (aliphatic) or closed-chain (cyclic); saturated (single bonds) vs unsaturated (double/triple)
●Homologous series: same general formula, differ by CH $_{2}$ , same functional group $\to$ similar chemistry, gradually changing physical properties
●Functional groups: $- O H$ (alcohol), $- C H O$ (aldehyde), $- C O -$ (ketone), $- C O O H$ (acid), $- C O O -$ (ester) — all with a carbonyl core except alcohol
●Structural isomers: same molecular formula, different arrangement $\to$ different properties (e.g. branching lowers boiling point)
●Alicyclic (ring of C only, e.g. cyclohexane) vs aromatic (benzene ring, delocalized, very stable) vs heterocyclic (ring with a non-C atom)
●Phenol ( $- O H$ on benzene ring) $\neq =$ alcohol: more acidic due to ring stabilizing the resulting negative charge

10th Class Chemistry Chapter 8 Notes

Organic Compounds and Their Classification

Homologous Series

Functional Groups

IUPAC Naming and Structural Formulae

Structural Isomerism

Ring (Closed-Chain) Compounds

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Carboxylic Acids

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