ORG CHEM | Structure, Naming, and Reactivity of Unsaturated Hydrocarbons

Note: Naming rules of alkanes and cycloalkanes are in its own reviewer. In addition, Disubstituted Benzenes, Carbocation Stability, Stereochemistry, and Reaction Conditions are not part of the test.

Alkenes

C_nH_2n, important in organic synthesis and industrial applications and are found in nature

Alkynes

C_nH_2n-2, used in the synthesis of complex molecules and in reactions

Aromatic Compounds

contains benzene rings or cyclic structures; its resonance creates a conjugated structure and makes these compounds very stable; prevalent in many natural and synthetic structures

Naming Alkenes and Alkynes (For Compounds with Only One Double or Triple Bond)

1. Identify the longest carbon chains that includes the double or triple bond.

2. Number the chain in such a way that the double or triple bond has the lowest possible number

3. Arrange the branches, just like you would for alkanes. Use the suffix “-ene” for alkenes and “-yne” for alkynes.

4. In between the prefix for the number of carbons in the parent chain (hep-, hex-, prop-, etc.) and the -ene/-yne suffix, put the number of the first carbon in the double or triple bond (e.g. 7-methyl-3-propyloct-1-ene)

Naming Alkenes and Alkynes (For Compounds with Both Double and Triple Bonds)

1. Identify the longest chain that includes both the double and triple bonds.

2. Test out the numbering where the double bond appears first and has the lowest possible position. Then, test out the numbering where the triple bond appears first and has the lowest possible position. Follow whichever numbering gives its respective multiple bond the lower position. If the multiple bonds have the same position, then follow the numbering where the double bond appears first.

3. When ordering the names, the “-en” (double bond) always appears before “-yne” (triple bond.) (e.g. hept-2-en-5-yne).

Naming Substituted Benzenes

Follow the same naming rules for cycloalkanes. However, use “benzene” as a suffix. Benzene will always be the parent chain for the test

Toulene

benzene with one methyl group

Xylene

benzene with 2 methyl groups

Naming Disubstituted Benzenes

Specify the positions of the substituents using either “ortho-” (substituents are at carbon 1 and 2), “meta-” (substituents are at carbon 1 and 3), and “para-” (substituents are at carbon 1 and 4),

Isomers

compounds with the same molecular formula but different structures

Constitutional/Structural Isomers

molecules with the same formula but different connectivity of atoms

Stereoisomers

molecules with the same connectivity but different spatial arrangement of atoms

Cis Isomers | Structure

stereoisomer where same side groups are placed on the same side of a double bond

Cis Isomers | Characteristics

usually polar, relatively lower melting points, high boiling point, soluble in inert solvents, higher acid power

Trans Isomers | Structure

stereoisomer where same side groups are placed on opposite sides of a double bond

Trans Isomers | Characteristics

nonpolar, higher melting points, lower boiling point, less soluble in neutral solvents, less acidic

Addition Reactions with an Alkene

When an alkene reacts to a compound, the reaction breaks up the double bond into a single bond. The elements of the compound each bond to a carbon that previously had the double bond.

Hydrogenation

An alkene reacts with hydrogen gas, breaking up the double bond and having a hydrogen atom attach to each of the carbons previously in the double bond. A metal catalyst initiates the reaction such as platinum, palladium, or nickel

Hydrogenation Process

The reactants adsorbed to the catalyst and a H atom bonds to a C atom to break up the double bond. The other C atom is still on the surface. When the H atom attaches to it, the new molecule leaves the surface.

Halogenation

An alkene reacts with halogens such as fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂), breaking up the double bond and having each atom attach to each of the carbons previously in the double bond.

Hydrohalogenation

An alkene reacts with a hydrogen halide such as HCl, HBr, and HI, breaking up the double bond and having each atom attach to one of the carbons previously in the double bond.

Hydrohalogenation in Symmetrical Alkenes

the two atoms of the hydrogen halide can bond to either of the atoms

Markovnikov’s Rule

In a hydrohalogenation reaction involving asymmetrical alkenes, the hydrogen atom will attach to the carbon atom with more hydrogen atoms previously attached while the halogen atom will attach to the carbon atom with less hydrogen atoms. The product where the opposite is true may be produced in a very small amount as a minor product.

Carbocation Stability

The more substituted the carbocation intermediate, the more stable it is, often leading to the halogen attaching to the more substituted carbon.

Stereochemistry

In some cases, the reaction may favor a specific stereochemical outcome, such as anti-Markovnikov addition or syn addition (the atoms are added to the same side of the bond).

Reaction Conditions

Factors like temperature, solvent, and catalyst can influence the product distribution.

Prefix for a Cl Atom Functional Group

chloro-

Prefix for a Br Atom Functional Group

bromo-

Prefix for a F Atom Functional Group

fluoro-

Prefix for a I Atom Functional Group

iodo-

Why do benzenes undergo substitution reactions instead of addition reactions?

The delocalization of its electrons due to its resonance surrounds the carbon atoms with an electron cloud. It is a source of electrons for electrophilic reagents, making electrophilic substitution reactions common.

Hydration

An alkene reacts with H2O. It also follows the Markovnikov’s rule, where the hydrogen atom bonds to the carbon atom with more hydrogen atoms while the hydroxyl group bonds to the carbon atom with less hydrogen atoms