Organic Functional Groups

Overview of Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing and understanding functional groups is essential for predicting the reactivity and properties of organic compounds.

• Alkyl halide (haloalkane): Contains a halogen atom (F, Cl, Br, I) bonded to an sp3 carbon.

• Alcohol: Contains a hydroxyl group (-OH) bonded to an sp3 carbon.

• Ether: Contains an oxygen atom bonded to two alkyl or aryl groups.

• Phosphate: Contains a phosphorus atom bonded to four oxygen atoms, one of which is double-bonded.

• Amine: Contains a nitrogen atom bonded to one or more alkyl or aryl groups.

• Thiol: Contains a sulfhydryl group (-SH) bonded to a carbon atom.

• Sulfide: Contains a sulfur atom bonded to two carbon atoms.

• Disulfide: Contains two sulfur atoms bonded together, each attached to a carbon atom.

• Aldehyde: Contains a carbonyl group (C=O) bonded to at least one hydrogen atom.

• Ketone: Contains a carbonyl group (C=O) bonded to two carbon atoms.

• Carboxylic acid: Contains a carbonyl group bonded to a hydroxyl group (-COOH).

• Ester: Contains a carbonyl group bonded to an oxygen atom, which is bonded to another carbon atom.

• Thioester: Contains a carbonyl group bonded to a sulfur atom, which is bonded to another carbon atom.

• Amide: Contains a carbonyl group bonded to a nitrogen atom.

• Acid chloride: Contains a carbonyl group bonded to a chlorine atom.

Alkanes and Isomerism

Structural (Constitutional) Isomers

Isomers are compounds with the same molecular formula but different structures. Structural isomers (constitutional isomers) differ in the connectivity of their atoms. They can vary by carbon skeleton, functional group, or the position of the functional group.

• Different carbon skeletons: Example: Butane (C4H10) and 2-methylpropane (isobutane).

• Different functional groups: Example: Ethanol (C2H6O) and dimethyl ether.

• Different positions of functional groups: Example: Isopropylamine and propylamine (C3H9N).

Physical Properties of Alkanes

Boiling and Melting Points

The boiling and melting points of alkanes increase with the number of carbon atoms due to greater van der Waals forces. Branching lowers boiling points because it reduces surface area and intermolecular interactions.

• Boiling point trend: Increases with chain length; decreases with branching.

• Melting point trend: Also increases with chain length, but less predictably due to packing effects.

Alkyl Groups and Nomenclature

Common Alkyl Groups

Alkyl groups are fragments of alkanes with one hydrogen removed, allowing them to attach to other atoms or groups. They are named based on the parent alkane.

• Isopropyl (i-Pr): 3-carbon group, (CH3)2CH–

• sec-Butyl (sec-Bu): 4-carbon group, CH3CH2CH(CH3)–

• Isobutyl: (CH3)2CHCH2–

• tert-Butyl (t-Bu): (CH3)3C–

• Neopentyl: (CH3)3CCH2–

Degree of Alkyl Substitution

Carbons in alkanes are classified by the number of other carbons to which they are attached:

• Primary (1°): Bonded to one other carbon.

• Secondary (2°): Bonded to two other carbons.

• Tertiary (3°): Bonded to three other carbons.

• Quaternary (4°): Bonded to four other carbons.

Hydrogen Classification

Hydrogens are also classified based on the carbon to which they are attached (primary, secondary, tertiary).

Conformations of Alkanes

Newman Projections and Conformational Analysis

Newman projections are used to visualize the spatial arrangement of bonds around a carbon-carbon single bond. They help in analyzing different conformations (staggered, eclipsed, gauche, anti) and their relative energies.

• Staggered conformation: Lowest energy; bonds are as far apart as possible.

• Eclipsed conformation: Highest energy; bonds are aligned, causing torsional strain.

• Gauche conformation: Staggered but with bulky groups 60° apart, causing steric strain.

• Anti conformation: Staggered with bulky groups 180° apart; most stable for butane.

Cyclic Alkanes

Physical Properties and Nomenclature

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in a ring. Their physical properties are similar to acyclic alkanes but are influenced by ring size and strain.

• General formula: CnH2n

• Naming: Prefix 'cyclo-' + alkane name; substituents are numbered to give the lowest possible numbers.

Cis-Trans Isomerism in Cycloalkanes

Cycloalkanes with two or more substituents can exhibit cis-trans (geometric) isomerism, depending on whether the substituents are on the same or opposite sides of the ring.

Ring Strain and Stability

Ring strain arises from angle strain, torsional strain, and steric strain. The stability of cycloalkanes depends on their ring size and the ability to adopt conformations that minimize strain.

• Angle strain: Deviation from ideal tetrahedral bond angles (109.5°).

• Torsional strain: Eclipsing interactions between bonds on adjacent atoms.

• Steric strain: Repulsion between atoms/groups that are too close.

Conformations of Cycloalkanes

Cyclopropane and cyclobutane are highly strained due to angle and torsional strain. Cyclopentane and cyclohexane are much less strained, with cyclohexane being nearly strain-free in its chair conformation.

• Cyclohexane: Adopts a chair conformation to minimize strain; can undergo ring flips that interchange axial and equatorial positions.

Axial and Equatorial Bonds

In the chair conformation of cyclohexane, each carbon has one axial (vertical) and one equatorial (slanted) bond. Substituents prefer the equatorial position to minimize 1,3-diaxial interactions (steric strain).

Ring Flip

A ring flip interconverts the two chair conformations of cyclohexane, exchanging axial and equatorial positions for all substituents.

Steric Strain in Monosubstituted Cyclohexanes

The 1,3-diaxial interaction causes steric strain when bulky groups occupy axial positions. The magnitude of this strain depends on the size of the substituent.

Bicyclic and Polycyclic Alkanes

Types of Bicyclic Compounds

Bicyclic alkanes contain two fused, bridged, or spirocyclic rings. Their nomenclature is based on the number of carbons in each bridge connecting the bridgehead carbons.

• Fused bicyclic: Two rings share two adjacent carbons (e.g., decalin).

• Bridged bicyclic: Two rings share non-adjacent carbons (e.g., norbornane).

• Spirocyclic: Two rings share a single carbon atom (spiro atom).

Additional info: The nomenclature for bicyclic alkanes uses the format bicyclo[a.b.c]alkane, where a, b, and c are the number of carbons in each bridge, listed in decreasing order.