Functional Groups and Key Organic Reactions: Structured Study Notes

Functional Groups in Organic Chemistry

Definition and Importance

Functional groups are specific atoms or groups of atoms arranged in a particular way within a molecule, imparting distinct physical and chemical properties. They are central to organic chemistry because they determine the reactivity and classification of organic compounds.

• Functional groups undergo the same types of reactions regardless of the molecular context.

• Functionalization refers to the addition of functional groups to a molecule to alter its properties.

• Functional groups help distinguish similar compounds and are key to chemical nomenclature.

Key Properties Determined by Functional Groups

• Bonding and shape

• Physical properties

• Type and strength of intermolecular forces

• Chemical reactivity

• Nomenclature

Bond Angles and Molecular Shapes

Ideal Bond Angles in Common Geometries

• Linear: 180°

• Trigonal Planar: 120°

• Tetrahedral: 109.5°

• Trigonal Bipyramidal: 120°, 90°

• Octahedral: 90°

Bond Angle Deviations

Not all bond angles are ideal; deviations occur due to lone pairs and differences in atomic sizes. For example, water (H2O) has a bond angle of 104.5°, while ammonia (NH3) has 107°.

Examples Table

Molecular Shape	Bond Angle	Examples
Linear	180°	BeF2, CO2, HCN
Trigonal Planar	120°	BF3, SO3
Tetrahedral	109.5°	CH4, SO42-
Trigonal Bipyramidal	90°, 120°	PF5, PCl5
Octahedral	90°	SF6, Mo(CO)6

Functional Groups: Heteroatoms and π Bonds

Role in Reactivity

• Heteroatoms (O, N, S, Cl, etc.) introduce lone pairs and create electron-deficient sites on carbon, increasing reactivity.

• π Bonds (C=C, C=O, C≡N) are easily broken and make molecules act as bases and nucleophiles.

Electronegativity and Inductive Effect

• Electronegativity measures an atom's ability to attract electrons. Differences in electronegativity create polar bonds and affect reactivity.

• Inductive Effect is the polarization of a σ bond due to electron donation or withdrawal by adjacent atoms/groups, influencing physical and chemical properties.

Classification of Functional Groups

Hydrocarbons and π Bonds

Type of Compound	General Structure	Example	Functional Group
Alkane	R-H	CH3CH3	None
Alkene	C=C	H2C=CH2	Double bond
Alkyne	C≡C	H-C≡C-H	Triple bond
Aromatic	Benzene ring	Phenyl group	Phenyl group

Heteroatoms

Type	General Structure	Example	Functional Group
Alkyl halide	R-X	CH3-Cl	Halo group
Alcohol	R-OH	CH3-CH2-OH	Hydroxy group
Ether	R-O-R	CH3-O-CH3	Alkoxy group
Amine	R-NH2	CH3-NH2	Amino group
Thiol	R-SH	CH3-SH	Mercapto group
Sulfide	R-S-R	CH3-S-CH3	Thio group

Carbonyl Group

Type	General Structure	Example	Functional Group
Aldehyde	R-CHO	CH3-CHO	Carbonyl group
Ketone	R-CO-R	CH3-CO-CH3	Carbonyl group
Carboxylic acid	R-COOH	CH3-COOH	Carboxyl group
Ester	R-COOR	CH3-COOCH3	Carboxyl group
Amide	R-CONH2	CH3-CONH2	Carboxyl group
Acid chloride	R-COCl	CH3-COCl	Carboxyl group

Functional Groups in Drugs and Natural Products

Multiple Functional Groups

• Many drugs (e.g., amoxicillin, atenolol, viracept) and natural flavors (e.g., vanilla, clove) contain multiple functional groups, which contribute to their biological activity and sensory properties.

IUPAC Nomenclature of Functional Groups

Class	Structure	IUPAC Suffix	IUPAC Prefix
Alkenes	C=C	-ene
Alkynes	C≡C	-yne
Halides	-Cl, -Br, -I		Halo
Alcohols	-OH	-ol	Hydroxy
Aldehydes	-CHO	-al	Oxo-
Ketones	C=O	-one	Oxo-
Nitriles	C≡N	-nitrile	Cyano
Ethers	R-O-R		Alkoxy
Carboxylic acid	-COOH	-oic acid	Carboxy
Carboxylate ion	-COO-	-oate
Esters	-COOR	-oate
Amines	-NH2	-amine	Amino
Amides	CONH2	-amide	Carbomoyl

Types of Organic Reactions

Overview

Organic reactions are chemical transformations involving organic compounds. The main types include:

• Substitution Reaction

• Elimination Reaction

• Addition Reaction

• Oxidation Reaction

Nucleophilic Substitution and Elimination Reactions of Alkyl Halides

Alkyl Halides

• Contain a halogen atom bonded to an sp3 hybridized carbon.

• Classified as primary (1°), secondary (2°), or tertiary (3°) based on the number of carbons attached to the halogen-bearing carbon.

• The polar C–X bond makes the carbon atom electron deficient and reactive.

Reaction Types

• Substitution with nucleophiles: $R-X + Nu^- ightarrow R-Nu + X^-$

• Elimination with bases: $R-X + B^- ightarrow$ (alkene) $+ H-B + X^-$

Nucleophilic Substitution Mechanisms

SN1 Mechanism (Unimolecular)

• Occurs in two steps: leaving group departs, then nucleophile attacks.

• Rate equation: $ext{Rate} = k[ ext{R–LG}]$

• First-order reaction; rate depends only on alkyl halide concentration.

• Carbocation intermediate is formed; stability of this intermediate determines reaction rate.

• Stereochemistry: produces a racemic mixture (50:50 enantiomers) due to planar carbocation.

SN2 Mechanism (Bimolecular)

• Occurs in one concerted step: nucleophile attacks as leaving group departs.

• Rate equation: $ext{Rate} = k[ ext{R–LG}][ ext{Nu}]$

• Second-order reaction; rate depends on both reactant concentrations.

• Stereochemistry: inversion of configuration at the reaction center.

Energy Diagrams and Reactivity

• SN1: Two transition states, carbocation intermediate.

• SN2: Single transition state, no intermediate.

• Factors affecting rate: substrate structure, nucleophile strength, solvent, and steric hindrance.

Comparison Table: SN1 vs SN2

Feature	SN1	SN2
Steps	Two	One
Intermediate	Carbocation	None
Order	First	Second
Stereochemistry	Racemic mixture	Inversion
Favored by	Weak nucleophile, tertiary substrate	Strong nucleophile, primary substrate

Resonance Stabilization of Carbocations

Allylic and Benzylic Carbocations

• Resonance stabilization increases carbocation stability, enhancing SN1 reactivity.

• Order of stability: methyl < primary < allylic < benzylic < secondary < tertiary.

Summary

• Functional groups define the reactivity and classification of organic compounds.

• Bond angles and molecular shapes are determined by electron pair geometry.

• Organic reactions include substitution, elimination, addition, and oxidation.

• Nucleophilic substitution occurs via SN1 or SN2 mechanisms, with distinct kinetics and stereochemical outcomes.

Additional info: These notes cover foundational topics in organic chemistry, including functional group identification, nomenclature, and key reaction mechanisms, suitable for college-level study.