Organic Chemistry: Structure, Properties, and Functional Groups

Organic Chemistry

Definition and Scope

Organic chemistry is the branch of chemistry that studies hydrocarbons and their derivatives. Over 95% of known chemical compounds are carbon-based, making organic chemistry a central field in science.

• Hydrocarbons: Compounds composed entirely of carbon and hydrogen.

• Derivatives: Compounds formed by substituting other atoms or groups for hydrogen in hydrocarbons.

The Unique Carbon Atom

Bonding and Structure

Carbon atoms can bond to each other, forming long chains and rings, which is the basis for the diversity of organic compounds.

• Catenation: The ability of carbon to form stable bonds with itself.

• Versatility: Enables the formation of a wide variety of structures, including straight chains, branched chains, and rings.

Hydrocarbons

Classification

Hydrocarbons are classified based on the types of bonds between carbon atoms.

• Alkanes: Only single bonds (saturated hydrocarbons).

• Alkenes: At least one double bond (unsaturated hydrocarbons).

• Alkynes: At least one triple bond (unsaturated hydrocarbons).

Alkanes

Structure and Nomenclature

Alkanes are saturated hydrocarbons with only single bonds. Their general formula is $C_nH_{2n+2}$.

• Naming: Prefix indicates the number of carbon atoms, suffix is '-ane'.

• Examples: Methane ($CH_4$), Ethane ($C_2H_6$), Propane ($C_3H_8$).

• Condensed Structural Formula: Shows the arrangement of atoms without drawing all bonds.

Stems for Organic Molecule Names

Stem	Number of Carbon Atoms
Meth-	1
Eth-	2
Prop-	3
But-	4
Pent-	5
Hex-	6
Hept-	7
Oct-	8
Non-	9
Dec-	10

Homologous Series and Isomerism

Alkanes form a homologous series, differing by a $-CH_2-$ group. Isomers have the same molecular formula but different structural arrangements.

• Isomerism: Structural isomers differ in connectivity.

Physical Properties, Uses, and Occurrences of Selected Alkanes

Name	Molecular Formula	Melting Point (°C)	Boiling Point (°C)	Density at 20°C (g/mL)	Use/Occurrence
Methane	CH4	-183	-162	Gas	Natural gas (main component); fuel
Ethane	C2H6	-172	-89	Gas	Natural gas (minor component); plastics
Propane	C3H8	-188	-42	Gas	LPG (bottled gas); plastics
Butane	C4H10	-138	0	Gas	LPG; lighter fuel
Pentane	C5H12	-130	36	0.626	Gasoline component
Hexane	C6H14	-95	69	0.659	Gasoline component; extraction solvent for food oils
Heptane	C7H16	-91	98	0.684	Gasoline component
Octane	C8H18	-57	126	0.703	Gasoline component

Cyclic Hydrocarbons

Structure and Nomenclature

Cyclic hydrocarbons are ring compounds. The simplest is cyclopropane. The prefix 'cyclo-' is used in naming.

Unsaturated Hydrocarbons

Alkenes

Alkenes contain at least one carbon-to-carbon double bond. Their general formula is $C_nH_{2n}$, and their names end with '-ene'.

• Example: Ethylene ($C_2H_4$)

Alkynes

Alkynes contain at least one carbon-to-carbon triple bond. Their general formula is $C_nH_{2n-2}$, and their names end with '-yne'.

• Example: Ethyne (acetylene, $C_2H_2$)

Cis–Trans Isomerism

Certain alkenes and cyclic hydrocarbons can exist as cis–trans isomers, a form of geometric isomerism where isomers have the same connectivity but differ in spatial orientation.

Aromatic Hydrocarbons

Benzene and Relatives

Benzene is a stable six-carbon ring. Aromatic hydrocarbons contain a benzene ring or have similar properties.

Chlorinated Hydrocarbons

Structure and Uses

Chlorinated hydrocarbons are formed by substituting chlorine for hydrogen in hydrocarbons. They are used as solvents, paint removers, and formerly as anesthetics.

• Dichloromethane: Solvent, paint remover

• Trichloromethane (chloroform): Solvent, hazardous anesthetic

Functional Groups

Definition and Importance

Functional groups are atoms or groups of atoms attached to hydrocarbon skeletons, giving compounds characteristic properties. Examples include double/triple bonds and halogen substituents.

Selected Organic Functional Groups

Name of Class	Functional Group	General Formula
Alkane	None	R-H
Alkene	C=C	R2C=CR2
Alkyne	C≡C	RC≡CR
Alcohol	–OH	R–OH
Ether	–O–	R–O–R'
Aldehyde	–CHO	R–CHO
Ketone	–CO–	R–CO–R'
Carboxylic acid	–COOH	R–COOH
Ester	–COOR'	R–COOR'
Amine	–NH2, –NHR, –NR2	R–NH2, R–NHR, R–NR2
Amide	–CONH2, –CONHR, –CONR2	R–CONH2, R–CONHR, R–CONR2

Alcohols

Structure and Examples

Alcohols contain the hydroxyl (–OH) functional group. Common examples include methanol, ethanol, and 1-propanol.

• Methanol: $CH_3OH$

• Ethanol: $CH_3CH_2OH$

• 1-Propanol: $CH_3CH_2CH_2OH$

Toxicity of Alcohols

All alcohols are toxic. Methanol is oxidized to formaldehyde, which is poisonous. Ethanol's toxicity causes inebriety, alcoholism, and fetal alcohol syndrome.

Multifunctional Alcohols

Some alcohols contain more than one hydroxyl group, such as ethylene glycol and glycerol.

Phenols

Structure and Properties

Phenols are aromatic compounds with a hydroxyl group attached to the ring. They act as acids and are effective antiseptics.

Ethers

Structure and Examples

Ethers have two alkyl groups bonded to the same oxygen atom. The general formula is $ROR$ or $ROR'$.

• Example: Diethyl ether ($CH_3CH_2OCH_2CH_3$)

Aldehydes and Ketones

Structure and Functional Group

Both contain the carbonyl group ($C=O$). Aldehydes have the group at the end of a chain, ketones have it within the chain.

Carboxylic Acids

Structure and Properties

Carboxylic acids contain the carboxyl group ($-COOH$). When attached to a benzene ring, the compound is called benzoic acid.

Esters

Formation and Properties

Esters are formed by replacing the hydrogen of a carboxylic acid with the alkyl group of an alcohol or phenol. They often have pleasant odors.

Ester Flavors and Fragrances

Ester	Formula	Flavor/Fragrance
Methyl butyrate	CH3CH2CH2COOCH3	Apple
Ethyl butyrate	CH3CH2CH2COOCH2CH3	Pineapple
Propyl acetate	CH3COOCH2CH2CH3	Pear
Pentyl acetate	CH3COOCH2CH2CH2CH2CH3	Banana
Pentyl butyrate	CH3CH2CH2COOCH2CH2CH2CH2CH3	Apricot
Octyl acetate	CH3COOCH2CH2CH2CH2CH2CH2CH2CH3	Orange
Methyl benzoate	C6H5COOCH3	Kiwifruit
Ethyl formate	HCOOCH2CH3	Rum
Methyl salicylate	o-HOC6H4COOCH3	Wintergreen
Benzyl acetate	CH3COOCH2C6H5	Jasmine

Amines and Amides

Amines

Amines are derivatives of ammonia, formed by replacing one or more hydrogens with alkyl groups. They are generally basic and have distinctive odors.

Amides

Amides have nitrogen bonded to a carbonyl carbon. They are important in biological molecules, such as proteins.

Heterocyclic Compounds

Structure and Significance

Heterocyclic compounds are rings containing atoms other than carbon, such as nitrogen, oxygen, or sulfur. Many are biologically important.

Alkaloids

Definition and Examples

Alkaloids are naturally occurring amines found in plants, bacteria, fungi, and animals. Many have physiological effects, including morphine, caffeine, nicotine, and cocaine. Pyrimidine and purine are also alkaloids.