Chapter 21: Introduction to Organic Chemistry

I. Introduction to Organic Chemistry

A. General

Organic chemistry is the study of compounds containing carbon. The simplest organic compounds are hydrocarbons, which contain only carbon and hydrogen. Hydrocarbons are classified into three main groups:

• Saturated hydrocarbons: Contain only single bonds between carbon atoms (alkanes and cycloalkanes).

• Unsaturated hydrocarbons: Contain one or more double or triple bonds between carbon atoms (alkenes and alkynes).

• Aromatic hydrocarbons: Contain benzene rings, characterized by alternating single and double bonds in a six-membered ring.

B. Classification of Hydrocarbons

Hydrocarbons are divided into two broad categories:

• Aliphatic hydrocarbons: Include alkanes, cycloalkanes, alkenes, and alkynes. Both saturated and unsaturated hydrocarbons are considered aliphatic.

• Aromatic hydrocarbons: Contain benzene rings and related structures.

II. Aliphatic Hydrocarbons

A. Alkanes

Alkanes are saturated hydrocarbons with the general formula $C_nH_{2n+2}$, where $n = 1, 2, 3, ...$. They contain only single covalent bonds and are the least reactive type of hydrocarbon.

• Structural formulas: The first four straight-chain alkanes are methane ($CH_4$), ethane ($C_2H_6$), propane ($C_3H_8$), and butane ($C_4H_{10}$).

Alkane	Molecular Formula	Melting Point (°C)	Boiling Point (°C)	Phase at STP	Number of Structural Isomers
Methane	CH4	-182.5	-161.5	gas	1
Ethane	C2H6	-183.3	-88.6	gas	1
Propane	C3H8	-187.7	-42.1	gas	1
Butane	C4H10	-138.3	-0.5	gas	2
Pentane	C5H12	-129.7	36.1	liquid	3
Hexane	C6H14	-95.0	68.7	liquid	5
Heptane	C7H16	-90.6	98.4	liquid	9
Octane	C8H18	-56.8	125.6	liquid	18

• Branched-chain alkanes: Alkanes can have straight or branched chains. For example, $C_4H_{10}$ can be drawn as either n-butane (straight chain) or isobutane (branched chain).

• Structural isomers: Compounds with the same molecular formula but different structural formulas. The number of isomers increases with the number of carbon atoms.

• Condensed structural formulas: Bonds around each carbon atom are not explicitly drawn (e.g., $CH_3CH_2CH_2CH_3$ for butane).

• Skeletal structures: Straight lines represent carbon-carbon bonds; carbon and hydrogen atoms are not explicitly shown.

Naming Branched-Chain Alkanes

1. Identify the longest continuous carbon chain (parent name).

2. Number the chain from the end giving the lowest numbers to branch points.

3. Name and locate each alkyl group (e.g., methyl, ethyl, propyl, butyl).

4. List alkyl groups in alphabetical order; use prefixes (di-, tri-, tetra-) for multiples of the same group.

Example: 2,2,4-trimethylpentane

B. Cycloalkanes

Cycloalkanes are alkanes whose carbon atoms are joined into rings. They have the general formula $C_nH_{2n}$, where $n = 3, 4, ...$ (e.g., cyclopropane, cyclobutane, cyclopentane, cyclohexane).

C. Alkenes

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond and the general formula $C_nH_{2n}$, where $n = 2, 3, ...$. The double bond increases reactivity compared to alkanes.

• Geometric isomers: Isomers with the same connectivity but different spatial arrangements due to restricted rotation around the double bond (e.g., cis-2-butene and trans-2-butene).

Naming Branched-Chain Alkenes

• The parent chain must contain the double bond, even if not the longest chain.

• Number from the end closest to the double bond.

• Include the position number of the double bond in the name.

Example: 4-methyl-2-pentene

Addition Reactions of Alkenes

Alkenes undergo addition reactions with reagents such as $H_2$, $Cl_2$, $Br_2$, $HCl$, $HBr$, and $H_2O$:

$C=C + X-Y \rightarrow H-C-C-H$

• Markovnikov's Rule: When adding HX to an unsymmetrical alkene, the hydrogen attaches to the carbon with more hydrogens already attached.

D. Alkynes

Alkynes are unsaturated hydrocarbons with at least one carbon-carbon triple bond and the general formula $C_nH_{2n-2}$, where $n = 2, 3, ...$. They are more reactive than alkanes and alkenes.

• Naming: The parent chain must contain the triple bond; number from the end closest to the triple bond and include its position in the name.

Example: 3-ethyl-1-heptyne

Addition Reactions of Alkynes

Alkynes undergo addition reactions similar to alkenes, with reagents such as $H_2$, $Cl_2$, $Br_2$, $HCl$, $HBr$, and $H_2O$.

III. Aromatic Hydrocarbons

A. General

Aromatic hydrocarbons contain benzene rings, which are six-membered rings with alternating single and double bonds. Benzene's structure is best represented by resonance structures.

B. Nomenclature

• Monosubstituted benzenes: Named by prefixing the substituent to 'benzene' (e.g., ethylbenzene, chlorobenzene, aminobenzene, nitrobenzene).

• Multisubstituted benzenes: Number the ring to give the lowest possible numbers to substituents.

C. Reactions

• Substitution reactions: The most common reaction for benzene. For example, bromination in the presence of iron(III) bromide ($FeBr_3$) catalyst:

$C_6H_6 + Br_2 \xrightarrow{FeBr_3} C_6H_5Br + HBr$

IV. Alcohols, Ethers, and Amines

A. Alcohols

• Alcohols are hydrocarbons in which an -OH group replaces a hydrogen atom. General formula: $R-OH$.

• Naming: Replace the '-e' ending of the parent alkane with '-ol' (e.g., methane → methanol).

• For alcohols with three or more carbons, indicate the position of the -OH group with a number.

• Classification: Primary (1°), secondary (2°), and tertiary (3°) alcohols, depending on the number of carbons attached to the carbon bearing the -OH group.

• Preparation: Alcohols can be prepared from alkenes by addition of water ($H_2O$).

B. Ethers

• Ethers contain the functional group $R-O-R'$, where R and R' are alkyl groups.

• Naming: Name the two alkyl groups in alphabetical order followed by 'ether' (e.g., ethyl methyl ether).

• Preparation: Formed by condensation of two alcohol molecules with elimination of water.

C. Amines

• Amines are derivatives of ammonia ($NH_3$) where one or more hydrogens are replaced by alkyl groups.

• Primary amines: $R-NH_2$; Secondary: $R_2NH$; Tertiary: $R_3N$.

• Naming: The simplest names end in 'amine' (e.g., methylamine).

V. Compounds with Carbonyl Groups

A. Aldehydes and Ketones

• Both contain the carbonyl group ($C=O$).

• Aldehydes: At least one hydrogen attached to the carbonyl carbon ($R-CHO$).

• Ketones: Two carbon atoms attached to the carbonyl carbon ($R-CO-R'$).

• Naming: Aldehydes use the suffix '-al', ketones use '-one'.

B. Carboxylic Acids

• Contain the functional group $R-COOH$.

• Naming: Use the suffix '-oic acid' (e.g., ethanoic acid).

C. Esters

• Functional group: $R-COOR'$.

• Naming: Based on the alcohol and acid from which they are derived; use the suffix '-oate'.

• Preparation: Produced by reacting a carboxylic acid with an alcohol, eliminating water.

• Equation: $R-COOH + R'-OH \rightarrow R-COOR' + H_2O$

D. Amides

• Contain nitrogen attached to the carbonyl group ($R-CONH_2$, $R-CONHR'$, $R-CONR'_2$).

• Naming: Use the suffix '-amide'.

• Preparation: Produced by reacting a carboxylic acid with an amine, eliminating water.

• Primary, secondary, and tertiary amides exist, depending on the number of alkyl groups attached to the nitrogen.

Summary/Review

• Hydrocarbons are classified as alkanes, alkenes, alkynes, and aromatic compounds.

• Functional groups such as alcohols, ethers, amines, carbonyls, carboxylic acids, esters, and amides define the reactivity and properties of organic molecules.

• Nomenclature rules allow systematic naming of organic compounds based on structure and functional groups.

Example Problem: Identify the functional groups present in a given compound by analyzing its structure for characteristic groups such as -OH, -COOH, -NH2, etc.

Additional info: This summary covers the foundational concepts of organic chemistry as presented in a general chemistry course, focusing on structure, classification, nomenclature, and basic reactions of organic molecules.