Basic Concept of Organic Chemistry

(i) CH$_3$–O–CH$_2$–CH$_2$–O–CH$_3$: the parent chain is ethane with a methoxy group on each carbon, so the name is 1,2-dimethoxyethane.

(ii) CHCl$_3$: methane with three chlorine substituents, so the name is trichloromethane (chloroform).

(i) $\mathrm{(CH_3)_2C=CH-CHO}$: the longest chain including the -CHO carbon has 4 carbons (C1 = CHO, C2 = CH, C3 = the carbon bearing the double bond, C4 = one CH$_3$). There is a C=C between C2 and C3 and one methyl branch on C3.

IUPAC name: 3-methylbut-2-enal

(ii) $\mathrm{COOH-COOH}$

IUPAC name: ethanedioic acid (commonly oxalic acid)

Homologous series

A homologous series is a series of organic compounds having the same functional group, in which successive members differ from one another by a –CH₂– unit (i.e. by a molecular mass of 14 amu), and which can be represented by one general formula. All members of the series show similar chemical properties and a gradual gradation in physical properties.

Characteristics of a homologous series

1. Same general formula. All members of a series can be represented by a single general formula. For example, alkanes: CₙH₂ₙ₊₂ (CH₄, C₂H₆, C₃H₈, …); alcohols: CₙH₂ₙ₊₁OH (CH₃OH, C₂H₅OH, C₃H₇OH, …).

2. Same functional group. Every member contains the same functional group, so all show similar chemical properties. For example, all alcohols contain the –OH group and all aldehydes contain the –CHO group.

3. Difference of one –CH₂– unit. Each successive member differs from the previous one by a –CH₂– group, i.e. by 14 amu in molecular mass. For example, CH₄ (16 amu) → C₂H₆ (30 amu) → C₃H₈ (44 amu).

4. Gradual change in physical properties. As molecular mass increases down the series, melting point, boiling point and density increase while solubility in water decreases. For example, in alcohols, methanol, ethanol and propanol are miscible with water, but higher members are only partly soluble.

5. General methods of preparation. All members of a homologous series can usually be prepared by the same general method. For example, all alcohols can be prepared by the hydration of alkenes or by the reduction of aldehydes and ketones.

A homologous series is a group of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a $\mathrm{-CH_2-}$ unit (14 u). Examples are the alkanes $\mathrm{CH_4, C_2H_6, C_3H_8}$ and the alcohols $\mathrm{CH_3OH, C_2H_5OH}$.

Characteristics:

1. All members can be represented by one general formula, e.g. $\mathrm{C_nH_{2n+2}}$ for alkanes, $\mathrm{C_nH_{2n+1}OH}$ for alcohols.
2. Successive members differ by $\mathrm{CH_2}$, i.e. by 14 in molecular mass.
3. All members contain the same functional group, so they show similar chemical properties and can be made by similar methods.
4. Physical properties change gradually with increasing molecular mass. Boiling point, melting point and density generally increase, and solubility in water decreases.
5. Members can be named by the same IUPAC rules, using the same suffix for the functional group.

(i) $\mathrm{HOOC-CH_2-CH_2-COOH}$ : Butanedioic acid (succinic acid).

(ii) $\mathrm{(CH_3)_2CH-CH=CH-COCl}$ : the chain is numbered from the COCl carbon (C1), the double bond is at C2, and the methyl group is at C4. Name: 4-methylpent-2-enoyl chloride.

(i) $\mathrm{(CH_3)_3C-OH}$: the longest chain containing the carbon bearing OH has 3 carbons (propane), with an $\mathrm{OH}$ and a methyl group both on C-2. IUPAC name: 2-methylpropan-2-ol.

(ii) $\mathrm{CH_3COOCH_3}$ is the ester of ethanoic acid and methanol. IUPAC name: methyl ethanoate.

Functional isomers have the same molecular formula but different functional groups.

Ethanoic acid (C$_2$H$_4$O$_2$, CH$_3$COOH, a carboxylic acid): functional isomer is the ester methyl methanoate, HCOOCH$_3$.

Propanone (C$_3$H$_6$O, CH$_3$COCH$_3$, a ketone): functional isomer is the aldehyde propanal, CH$_3$CH$_2$CHO.

Ethanoic acid (acetic acid) has molecular formula $\mathrm{C_2H_4O_2}$ and structural formula:

$\mathrm{CH_3{-}COOH}$

Its functional isomer is methyl methanoate, an ester:

$\mathrm{H{-}COO{-}CH_3}$

IUPAC name of the isomer: methyl methanoate.

Homologous series: a group of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a $-\mathrm{CH_2}$ unit and can be represented by a general formula.

Series with –CHO (aldehydes): general formula $\mathrm{C_nH_{2n+1}CHO}$: $\mathrm{HCHO}$ (methanal) → $\mathrm{CH_3CHO}$ (ethanal) → $\mathrm{C_2H_5CHO}$ (propanal) → $\mathrm{C_3H_7CHO}$ (butanal).

Characteristics

1. Successive members differ by $-\mathrm{CH_2}$ (14 mass units).
2. All members have the same functional group and similar chemical properties, and can be made by similar methods.
3. Physical properties (boiling point, density) change gradually with increasing molar mass.

(c) Carboxylic acid

The group $-\mathrm{COOH}$ attached to the carbon chain is the carboxyl group, so the compound $\mathrm{CH_3CH_2COOH}$ (propanoic acid) belongs to the carboxylic acid family.

IUPAC names:

(i) $\mathrm{CH_3-CH_2-CH_2-CH_3}$: a straight chain of 4 carbon atoms with all single bonds (alkane).

• Parent: butane
• No branching, no multiple bond, no functional group

Name: Butane

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(ii) $\mathrm{CH_3-CH=CH-CH_3}$: 4 carbon chain containing a double bond between C-2 and C-3.

• Parent: butene
• Numbering from either end nearer the double bond gives the double bond at position 2.

Name: But-2-ene

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(iii) $\mathrm{CH_3-CH_2-OH}$: 2 carbon chain with an alcohol (–OH) group.

• Parent: ethane → replace final 'e' by 'ol' → ethanol
• –OH is on C-1, so no locant is needed.

Name: Ethanol

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(iv) $\mathrm{CH_3-CO-CH_3}$: 3 carbon chain with a carbonyl (ketone) group at C-2.

• Parent: propane → replace final 'e' by 'one' → propanone
• The $\mathrm{C=O}$ is at position 2.

Name: Propan-2-one (also called acetone)

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(v) $\mathrm{CH_3-CH_2-CHO}$: 3 carbon chain ending in an aldehyde (–CHO) group.

• Parent: propane → replace final 'e' by 'al' → propanal
• The –CHO carbon is always C-1, so no locant is needed.

Name: Propanal

Structural Isomerism

Structural isomerism (also called constitutional isomerism) is the phenomenon in which two or more compounds have the same molecular formula but different structural formulae, i.e. their atoms are linked together in a different order. Because the connectivity differs, such isomers have different physical and usually different chemical properties.

Example: Both ethanol and dimethyl ether have the molecular formula $\mathrm{C_{2}H_{6}O}$:

• Ethanol: $\mathrm{CH_{3}-CH_{2}-OH}$
• Dimethyl ether: $\mathrm{CH_{3}-O-CH_{3}}$

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Types of Structural Isomerism

1. Chain (skeletal) isomerism

The isomers differ in the arrangement of the carbon skeleton (straight chain or branched chain), while the molecular formula remains the same.

Example: $\mathrm{C_{4}H_{10}}$ (butane) exists as two chain isomers:

• $n$-Butane: $\mathrm{CH_{3}-CH_{2}-CH_{2}-CH_{3}}$ (straight chain)
• Isobutane: $\mathrm{CH_{3}-CH(CH_{3})-CH_{3}}$ (branched chain)

2. Position isomerism

The isomers differ in the position of the same functional group, substituent or multiple bond on the same carbon skeleton.

Example: $\mathrm{C_{3}H_{8}O}$ (propanol) exists as:

• Propan-1-ol: $\mathrm{CH_{3}-CH_{2}-CH_{2}-OH}$
• Propan-2-ol: $\mathrm{CH_{3}-CH(OH)-CH_{3}}$

Similarly, $\mathrm{C_{4}H_{8}}$ gives but-1-ene ($\mathrm{CH_{2}=CH-CH_{2}-CH_{3}}$) and but-2-ene ($\mathrm{CH_{3}-CH=CH-CH_{3}}$).

3. Functional group isomerism

The isomers have the same molecular formula but contain different functional groups, so they belong to different homologous series.

Example: $\mathrm{C_{3}H_{6}O}$ exists as:

• Propanal (aldehyde): $\mathrm{CH_{3}-CH_{2}-CHO}$
• Propanone (ketone): $\mathrm{CH_{3}-CO-CH_{3}}$

Another example: $\mathrm{C_{2}H_{6}O}$ gives ethanol (alcohol) and dimethyl ether (ether).

4. Metamerism

The isomers differ in the distribution of carbon atoms on either side of the functional group, i.e. the alkyl groups attached to the same functional group are different. It is shown by ethers, ketones, esters, etc.

Example: $\mathrm{C_{4}H_{10}O}$ (ethers) gives:

• $\mathrm{CH_{3}-O-CH_{2}-CH_{2}-CH_{3}}$ (methyl propyl ether)
• $\mathrm{CH_{3}-CH_{2}-O-CH_{2}-CH_{3}}$ (diethyl ether)

5. Tautomerism

A special type of functional isomerism in which the isomers exist in dynamic equilibrium with each other and differ in the position of a hydrogen atom and a double bond (keto–enol form).

Example: Acetoacetic ester exists as:

$\mathrm{CH_{3}-CO-CH_{2}-COOC_{2}H_{5} \rightleftharpoons CH_{3}-C(OH)=CH-COOC_{2}H_{5}}$

(keto form) $\rightleftharpoons$ (enol form)

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Distinction between Saturated and Unsaturated Hydrocarbons

Thus structural isomerism arises due to different modes of linking of atoms in the same molecular formula, and its main types are chain, position, functional group, metamerism and tautomerism.