Alkanes: Properties, Naming, and Reactions

Alcanes

• Consist of carbon and hydrogen atoms linked by single bonds.
• They are nonpolar molecules.
• Relatively unreactive but can undergo combustion and halogenation.

Naming Alkanes

• Identify the longest continuous chain of carbon atoms.
• Number the longest chain starting nearest to a substituent.
• Name each substituent as an alkyl group.
• Group identical substituents using prefixes like di-, tri-, tetra-.
• List substituents alphabetically.
• Place position numbers before the substituent names.
• Combine all elements into a single name.

Properties of Alkanes

• Soluble in nonpolar solvents due to their nonpolarity.
• Boiling points increase with increasing molecular weight and branched alkanes have lower boiling points.
• Melting points increase with increasing molecular weight and alkanes with greater symmetry have higher melting points.
• Density increases with increasing molecular weight and are generally less dense than water.

Reactions of Alkanes

• Combustion involves reaction with oxygen to form carbon dioxide, water, and heat.
  • $C_nH_{2n+2} + O_2 \rightarrow CO_2 + H_2O$
• Halogenation involves reaction with halogens (Cl2, Br2) in the presence of light or heat to form haloalkanes.
  • $R-H + X_2 \rightarrow R-X + HX$

Isomerism in Alkanes

• Constitutional isomers have the same molecular formula but different connectivity.
• Conformational isomers have different spatial arrangements due to rotation around single bonds.

Alkenes

• Contain at least one carbon-carbon double bond.
• More reactive than alkanes due to the double bond.

Naming Alkenes

• Identify the longest chain containing the double bond.
• Number the chain so the double bond has the lowest possible number.
• Name substituents as with alkanes.
• Indicate the double bond position before the alkene name.
• Use prefixes like di-, tri- if there are multiple double bonds.
• Identify cis and trans isomers.

Properties of Alkenes

• Soluble in nonpolar solvents due to their nonpolarity.
• Boiling points are similar to alkanes with comparable molecular weight.
• Are more reactive than alkanes due to the double bond's presence.

Reactions of Alkenes

• Addition reactions are common.
  • Hydrogenation: Addition of H2 to form alkanes.
    • $RCH=CH_2 + H_2 \rightarrow RCH_2CH_3$
  • Halogenation: Addition of halogens (Cl2, Br2) to form dihaloalkanes.
    • $RCH=CH_2 + X_2 \rightarrow RCHXCH_2X$
  • Hydrohalogenation: Addition of HX (HCl, HBr) to form haloalkanes, following Markovnikov's rule.
    • $RCH=CH_2 + HX \rightarrow RCHXCH_3$
  • Hydration: Addition of water (H2O) to form alcohols, following Markovnikov's rule.
    • $RCH=CH_2 + H_2O \rightarrow RCH(OH)CH_3$
  • Polymerization involves forming polymers from alkene monomers.

Isomerism in Alkenes

• Geometric isomerism (cis/trans) exists due to restricted rotation around the double bond.
• E/Z isomers are used when groups on the double bond are different.

Alkynes

• Contain at least one carbon-carbon triple bond.
• Are more reactive than alkanes and alkenes due to the triple bond.

Naming Alkynes

• Identify the longest chain containing the triple bond.
• Number the chain such that the triple bond gets the lowest possible number.
• Name substituents as with alkanes.
• Indicate the triple bond's position with a number before the alkyne name.
• Use prefixes like di-, tri- if there are multiple triple bonds.

Properties of Alkynes

• Soluble in nonpolar solvents due to their nonpolarity.
• Boiling points are similar to alkanes and alkenes with comparable molecular weight.
• Are more reactive than alkanes and alkenes because of the triple bond.

Reactions of Alkynes

• Addition reactions are key.
  • Hydrogenation: Addition of H2 to form alkanes or alkenes.
    • $RC \equiv CH + H_2 \rightarrow RCH=CH_2$
    • $RCH=CH_2 + H_2 \rightarrow RCH_2CH_3$
  • Halogenation: Addition of halogens (Cl2, Br2) to form tetrahaloalkanes.
    • $RC \equiv CH + X_2 \rightarrow RCX=CHX$
    • $RCX=CHX + X_2 \rightarrow RCX_2CHX_2$
  • Hydrohalogenation: Addition of HX (HCl, HBr) to form haloalkenes, following Markovnikov's rule.
    • $RC \equiv CH + HX \rightarrow RCX=CH_2$
  • Hydration: Addition of water (H2O) to form ketones or aldehydes, following Markovnikov's rule.
    • $RC \equiv CH + H_2O \rightarrow RC(=O)CH_3$
  • Alkylation involves reaction with strong bases to form alkynide anions, which can react with haloalkanes.

Isomerism in Alkynes

• Positional isomers differ in the triple bond's position on the chain.
• Do not exhibit geometric isomerism (cis/trans).

Diagrams

• Alkanes: Linear and branched structures (e.g., methane, ethane, propane, butane, isobutane, pentane, isopentane, neopentane).
• Alkenes: Structures showing the double bond and cis/trans isomerism (e.g., ethene, propene, but-2-ene).
• Alkynes: Triple bond illustrations (e.g., ethyne/acetylene, propyne, but-2-yne).