Chemistry 20 Chapter 2 Alkanes

Carbon Carbon has four valence electrons; hydrogen has one. • Review Carbon Carbon has four valence electrons; hydrogen has one. • • C • H • To obtain an octet, carbon forms four bonds. H H • • H  C  H H C H CH4 , methane H H H

Hydrocarbons Large family of organic compounds Composed of only carbon and hydrogen Saturated hydrocarbons Alkanes Unsaturated hydrocarbons Alkenes, Alkynes & Aromatics C - C C = C C C

Alkanes

Expanded structural formula (Lewis): Methane Tetrahedral Expanded structural formula (Lewis): showing each bond line. CH4 Molecular formula

Ethane C2H6 Molecular formula Expanded structural formula CH3 – CH3 Condensed structural formula: with each carbon atom and its attached hydrogen atoms.

Alkanes CnH2n+2 n: number of carbon atoms

Naming of Alkanes CnH2n+2 Prefix + ane

Line-angle Formula Propane CH3-CH2-CH3 Butane CH3-CH2-CH2-CH3 Pentane CH3-CH2-CH2-CH2-CH3

Naming Substituents In the IUPAC system: Removing a H from an alkane is called alkyl group. -ane Halogen atoms are named as halo. -ine -yl -O -OH Hydroxyl -NO2 Nitro

Number the carbon atoms starting from the end nearest a substituent. STEP 2 Number the carbon atoms starting from the end nearest a substituent. STEP 3 Give the location and name of each substituent (alphabetical order) as a prefix to the name of the main chain.

Give the name of: CH3  CH3─CH─CH2─CH3 STEP 1 Longest chain is butane. STEP 2 Number chain. CH3 1 2 3 4 STEP 3 Locate substituents and name. 2-Methylbutane

Give the name of: CH3 CH3   CH3─CH─CH─CH3 STEP 1 Longest chain is butane. STEP 2 Number chain. CH3 CH3   1 2 3 4 STEP 3 Locate substituents and name. 2,3-dimethylbutane

Cl CH3   CH3─CH2─CH─CH─CH3 STEP 1 Longest chain is pentane. STEP 2 Number chain from end nearest substituent. Cl CH3   CH3─CH2─CH─CH─CH3 5 4 3 2 1 STEP 3 Locate substituents and name alphabetically. 3-Chloro-2-methylpentane

CH3 CH3 | | CH3─CH─CH2 ─CH─CH3 2,4-dimethylpentane 1 2 3 4 5 Cl CH3 CH3─CH2─CH─CH2─C─CH2─CH3 | Cl 7 6 5 4 3 2 1 3,5-dichloro-3-methylheptane

STEP 1 Longest chain has 5 carbon atoms (Pentane). CH2 CH3  CH3─CH─CH2─CH3 STEP 1 Longest chain has 5 carbon atoms (Pentane). STEP 2 Number chain from end nearest substituent. STEP 3 Locate substituent and name. 3-Methylpentane 2 1 3 4 5

Constitutional Isomers Have the same molecular formula. Have different atom arrangements (different structural formula). CH3CH2CH2CH3 Butane C4H10 CH3 2-Methylpropane CH3CHCH3 C4H10

Cyclic Hydrocarbon - Cycloalkane Carbon atoms are joined to form a ring. Cyclobutane = = Cyclopentane = Cyclohexane

Naming of Cycloalkanes Prefix “cyclo-” + the name of the open-chain alkane Substituents: One substituent: no location number Two or more substituents: number the ring beginning with the substituent of lower alphabetical order. Cl CH3 Br NO2 OH 1 2 3 Chlorocyclohexane 1-Bromo-2-methylcyclohexane 1-Chloro-3-hydroxyl-2-nitrocyclopentane

Conformation - 3D shapes Any three-dimensional arrangement of atoms in a molecule that results by rotation about a single bond. The following are three conformations for a butane molecule. 1 2 3 4

Conformation - 3D shapes The most stable conformation of a cyclopentane is the envelope conformation. The most stable conformation of a cyclohexane is the chair conformation. All bond angles are approximately 109.5°.

Conformation - 3D shapes In a chair conformation, six C-H bonds are equatorial bonds. six C-H bonds are axial bonds. Equatorial Axial

Conformation - 3D shapes The more stable conformation of a substituted cyclohexane ring has the substituent group(s) as equatorial rather than axial. H H H H More Stable

Cis & Trans Stereoisomers The same molecular formula and the same connectivity of their atoms, but a different arrangement of their atoms in space. mp & bp of cis < mp & bp of trans

Cis & Trans Stereoisomers Stereoisomers: Isomers that have the same molecular formulas and the same connectivity of their atoms but a different orientation of their atoms in space.

Physical Properties of Alkanes Nonpolar Insoluble in water. Lower density than water. Low boiling and melting points. Gases with 1-4 carbon atoms. (methane, propane, butane) Liquids with 5-17 carbon atoms. (kerosene, diesel, and jet fuels) Solids with 18 or more carbon atoms. (wax, paraffin, Vaseline)

Boiling & melting points of Alkanes Number of carbon atoms ↑ bp & mp ↑ Number of branches ↑ bp & mp ↓ CH3 CH3CH2CH2CH3 CH3CHCH3

Chemical reactions of Alkanes Low reactivity 1- Combustion: Alkanes react with oxygen. CO2, H2O, and energy are produced. Alkane + O2 CO2 + H2O + heat CH4 + 2O2 CO2 + 2H2O + energy

Chemical reactions of Alkanes Low reactivity 2- Halogenation: Alkanes react with Halogens. CH4 + Cl2 CH3Cl + HCl Chloromethane Heat or light Dichloromethane CH3Cl+ Cl2 CH2Cl2 + HCl Heat or light CH2Cl2 + Cl2 CHCl3 + HCl Trichloromethane Heat or light CHCl3 + Cl2 CCl4 + HCl Tetrachloromethane Heat or light

Sources of Alkanes Natural gas 90 to 95 percent methane. 5 to 10 percent ethane, and a mixture of other low-boiling alkanes, chiefly propane, butane, and 2-methylpropane. Petroleum A thick liquid mixture of thousands of compounds, most of them hydrocarbons, formed from the decomposition of marine plants and animals.