Lecture Notes #4

Functional Groups and Intermolecular Forces

Learning Goals

• Identify the major functional groups in organic molecules.

• Understand how functional groups participate in intermolecular forces and influence molecular properties.

Functional Groups

Definition and Importance

Functional groups are specific groups of atoms within molecules that are responsible for characteristic chemical reactions. They define the reactivity and properties of organic compounds.

• Consist of: alkenes, aromatic rings, alkynes, and heteroatoms (usually N, O, F, Cl, Br, I, P, S).

• Heteroatoms are drawn explicitly in line-bond structures; lone pairs are implied to satisfy the octet rule.

Classification of Functional Groups

Halides (R–X)

Halides are organic compounds where a halogen (X) is bonded to a carbon (R).

• Halogens: F, Cl, Br, I (each has 3 implied lone pairs).

• Examples: methyl chloride (MeCl), tert-butyl bromide (tBuBr), aryl fluoride (PhF).

Oxygen-Containing Groups

• Alcohols (R–OH): Hydroxyl group bonded to carbon. Example: ethanol, isopropyl alcohol.

• Ethers (R–O–R'): Oxygen atom bonded to two carbons. Example: diethyl ether (Et2O), methyl tert-butyl ether (MeOtBu).

Carbonyl Groups (C=O)

Carbonyl groups are characterized by a carbon double-bonded to oxygen. The nature of the R group determines the specific functional group.

• Aldehydes (R–CHO): R is H or alkyl. Example: acetaldehyde.

• Ketones (R–CO–R'): R and R' are alkyl or aryl. Example: methyl ethyl ketone.

• Carboxylic Acids (R–COOH): Contains a carboxyl group. Example: acetic acid.

• Esters (R–COOR'): Derived from carboxylic acids and alcohols. Example: ethyl acetate.

• Amides (R–CONH2): Contains a carbonyl attached to nitrogen. Example: acetamide.

Nitrogen-Containing Groups

• Amines (R–NH2, R2NH, R3N): Nitrogen bonded to one, two, or three carbons. Example: butyl amine, methyl propyl amine, triethyl amine.

• Nitriles (R–C≡N): Cyano group. Example: allyl nitrile.

Sulfur-Containing Groups

• Thiols (R–SH): Sulfur analogue of alcohols. Example: isopropyl thiol.

• Thioethers (R–S–R'): Sulfur atom bonded to two carbons. Example: dibenzyl sulfide.

Polarity and Bond Dipoles

Electronegativity and Bond Polarity

Polarity arises when atoms in a bond have different electronegativities, causing unequal sharing of electrons.

• Most heteroatoms are more electronegative than carbon.

• Polar bonds: e.g., C–O, O–H, C–F.

Example bond energies:

• C–C: ~346 kJ/mol

• C–F: ~485 kJ/mol

• C=O: ~750 kJ/mol

Depicting Bond Dipoles

• Draw dipole arrows towards the more electronegative atom.

• Presence of a polar bond does not guarantee a polar molecule; net dipole is required.

• Greater molecular polarity leads to stronger intermolecular forces.

Intermolecular Forces (IMFs)

Types of Intermolecular Forces

• Van der Waals / London Dispersion Forces: Weak (0.5–4 kJ/mol), arise from temporary induced dipoles. Larger molecules have stronger dispersion forces.

• Dipole-Dipole Interactions: Attraction between permanent dipoles in polar molecules. Example: acetone (bp = -24°C) vs. nonpolar molecules (bp = -44°C).

• Hydrogen Bonding: Strongest (5–30 kJ/mol), occurs when H is bonded to N, O, or F and interacts with lone pairs on another N, O, or F atom. Both donor and acceptor atoms can participate.

Solvents and Solubility

Solvent Types and Their Properties

Solvents dissolve solutes based on the strength of intermolecular forces between them.

Type	Examples	Properties
Nonpolar	Benzene, Toluene	Cannot dissolve polar solutes; weak IMFs
Polar Aprotic	Acetone, Acetonitrile, DMF	No O–H or N–H bonds; cannot donate H-bonds; good for dissolving ions
Polar Protic	Water, Methanol, Ethanol	Can donate H-bonds; best for dissolving ions

Applications: Functional Groups in Pharmaceuticals

Statins and Binding Sites

Functional groups in statins (e.g., atorvastatin, rosuvastatin) enable binding to the active site of HMG CoA-Reductase, inhibiting cholesterol production. The presence and arrangement of functional groups determine the strength and specificity of these interactions.

Practice Questions

Identifying Functional Groups

• Given a molecular structure, identify which functional groups are present or absent (e.g., alcohol, amine, nitrile).

Bond Polarity

• Of the bonds listed (C=O, C–H, O–H, O–O), the most polar is O–H due to the large electronegativity difference between O and H.

Summary Table: Functional Groups and Their Properties

Functional Group	General Formula	Key Atom(s)	Example
Alcohol	R–OH	O	Ethanol
Amine	R–NH2	N	Butyl amine
Halide	R–X	F, Cl, Br, I	Methyl chloride
Ketone	R–CO–R'	C=O	Acetone
Carboxylic Acid	R–COOH	C=O, O–H	Acetic acid
Amide	R–CONH2	C=O, N	Acetamide
Nitrile	R–C≡N	N	Allyl nitrile
Thiols	R–SH	S	Isopropyl thiol

Key Equations

• Bond Polarity: $\Delta EN = |EN_{atom1} - EN_{atom2}|$

• Hydrogen Bonding Strength: $5-30\ \text{kJ/mol}$

• London Dispersion Force Strength: $0.5-4\ \text{kJ/mol}$

Additional info: Expanded explanations and examples were added for completeness and clarity. Tables were inferred and constructed to summarize key points and properties.