Chapter 4: Structures and Functional Groups in Organic Compounds

Representing the Structures of Organic Compounds

Types of Structural Representations

Organic compounds can be represented in several ways, each providing different levels of detail about the molecule's structure.

• Molecular Formula: Shows the number and type of atoms present (e.g., C3H8).

• Condensed Structure: Groups atoms to show connectivity but omits most bonds (e.g., CH3CH2CH3).

• Lewis Structure: Displays all atoms and bonds explicitly, including lone pairs.

• Skeletal Structure: Simplifies organic molecules by representing carbon atoms as vertices and omitting hydrogen atoms bonded to carbon.

• Ball-and-Stick Model: A 3D representation showing spatial arrangement of atoms and bonds.

Example: Propane can be represented as:

• Molecular formula: C3H8

• Condensed: CH3CH2CH3

• Lewis: Shows all bonds and atoms

• Skeletal: Three connected lines (vertices = carbons)

Alkanes and Cycloalkanes

Linear and Branched Alkanes

Alkanes are saturated hydrocarbons containing only single bonds between carbon atoms. Their general formula is $C_nH_{2n+2}$ for acyclic (open-chain) alkanes.

Prefix	Name of Alkane	Molecular Formula	Condensed Structure	Skeletal Structure
meth-	Methane	CH4	CH4	(single point)
eth-	Ethane	C2H6	CH3CH3	—
prop-	Propane	C3H8	CH3CH2CH3	— —
but-	Butane	C4H10	CH3CH2CH2CH3	— — —
pent-	Pentane	C5H12	CH3CH2CH2CH2CH3	— — — —
hex-	Hexane	C6H14	CH3CH2CH2CH2CH2CH3	— — — — —
hept-	Heptane	C7H16	CH3CH2CH2CH2CH2CH2CH3	— — — — — —
oct-	Octane	C8H18	CH3CH2CH2CH2CH2CH2CH2CH3	— — — — — — —
non-	Nonane	C9H20	CH3CH2...CH3	— — — — — — — —
dec-	Decane	C10H22	CH3CH2...CH3	— — — — — — — — —

Cycloalkanes

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring. Their general formula is $C_nH_{2n}$.

Name	Molecular Formula	Structure
Cyclobutane	C4H8	Square ring
Cyclopentane	C5H10	Pentagon ring
Cyclohexane	C6H12	Hexagon ring

Physical Properties of Alkanes

• Nonpolarity: Alkanes are nonpolar due to similar electronegativities of C and H and their symmetrical structures.

• Common Uses: Alkanes are found in natural gas, propane gas, gasoline, diesel fuel, and candle wax (paraffin).

Alkane	Where Commonly Found	Chemical Structure
Methane (CH4)	Natural gas	CH4
Propane (C3H8)	Propane gas	Skeletal: three carbons
Octane (C8H18)	Gasoline	Branched chain
Hexadecane (C16H34)	Diesel fuel	Long straight chain
Octacosane (C28H58)	Candle wax	Very long chain

Types of Hydrocarbons

Classification by Bond Type

Type	Functional Group
Alkane	None; only C and H single bonds
Alkene	C=C double bond
Alkyne	C≡C triple bond
Aromatic	Planar, ring structures based on benzene; may contain heteroatoms (N, O, S)

Families of Organic Compounds: Functional Groups

Overview of Functional Groups

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules.

Family Name	Representative Structure	Example of Compound Containing Functional Group
Alkane	All C–C bonds are single	Ethane
Alkene	Contains C=C double bonds	Ethene
Alkyne	Contains C≡C triple bonds	Ethylene
Aromatic	Benzene ring	Benzene
Alcohol	R–OH	Ethanol
Phenol	Aromatic ring–OH	Phenol
Ether	R–O–R'	Diethyl ether
Thiol	R–SH	Cysteine
Sulfide	R–S–R'	Dimethyl sulfide
Disulfide	R–S–S–R'	Disulfide bridges in proteins
Phosphate	–PO4	ATP
Amine	R–NH2, R2NH, R3N	Amphetamine
Amide	R–CO–NH2	Peptide bond
Acetyl	R–CO–CH3	Acetyl coenzyme A
Aldehyde	R–CHO	Benzaldehyde
Ketone	R–CO–R'	Acetone
Carboxylic acid	R–COOH	Oleic acid
Carboxylate	R–COO–	Carboxylate ion
Ester	R–COO–R'	Triglyceride

Examples of Functional Groups in Biologically Important Molecules

• Morphine, Oxycodone, Tramadol, Naloxone: These molecules contain multiple functional groups such as alcohols, ethers, amines, and aromatic rings, which contribute to their biological activity.

Fatty Acids and Lipids

Structures of Common Lipids

Fatty acids are carboxylic acids with long hydrocarbon chains. The number of carbon atoms and the presence of double bonds determine their properties.

Carbon Atoms	Source	Structure
12	Coconut	Straight chain with 12 carbons, terminal carboxylic acid
14	Nutmeg	Straight chain with 14 carbons, terminal carboxylic acid
16	Palm	Straight chain with 16 carbons, terminal carboxylic acid
18	Animal fat	Straight chain with 18 carbons, terminal carboxylic acid
20	Peanut	Straight chain with 20 carbons, terminal carboxylic acid
22	Canola	Straight chain with 22 carbons, terminal carboxylic acid

Saturated vs. Unsaturated Lipids

• Saturated fatty acids: Contain only single bonds between carbon atoms (alkane chains).

• Unsaturated fatty acids: Contain one or more double bonds (alkene groups) in the hydrocarbon chain.

• Triglycerides: Fats composed of three fatty acid chains esterified to a glycerol backbone. Can be saturated or unsaturated.

Example: Olive oil is rich in unsaturated fatty acids, while butter contains more saturated fatty acids.

Isomerism in Organic Compounds

Structural and Stereoisomers

• Structural (Constitutional) Isomers: Compounds with the same molecular formula but different connectivity of atoms.

• Conformational Isomers: Same connectivity, differ by rotation around single bonds.

• Stereoisomers: Same connectivity, different spatial arrangement of atoms.

Cis-Trans (Geometric) Isomerism

• Cis isomer: Substituents on the same side of a double bond or ring.

• Trans isomer: Substituents on opposite sides.

Example: In 2-butene, cis-2-butene has both methyl groups on the same side of the double bond, while trans-2-butene has them on opposite sides.

Chirality and Enantiomers

• Chiral center: A carbon atom bonded to four different groups.

• Enantiomers: Non-superimposable mirror images; have identical physical properties except for the direction in which they rotate plane-polarized light and their reactions with other chiral substances.

• Determining chirality: Look for carbons with four different substituents.

Example: Lactic acid has a chiral center at the central carbon atom.

Summary Table: Types of Isomerism

Type	Description	Example
Structural isomers	Different connectivity	Butane vs. isobutane
Conformational isomers	Rotation about single bonds	Staggered vs. eclipsed ethane
Cis-trans isomers	Different arrangement around double bond	cis-2-butene vs. trans-2-butene
Enantiomers	Non-superimposable mirror images	L- and D-glucose

Key Equations and Concepts

• General formula for alkanes: $C_nH_{2n+2}$

• General formula for cycloalkanes: $C_nH_{2n}$

• General formula for alkenes: $C_nH_{2n}$

• General formula for alkynes: $C_nH_{2n-2}$

Additional info: The notes also reference the importance of functional groups in biological molecules, such as amino acids, nucleotides, and neurotransmitters, and their role in determining molecular properties and reactivity.