Chemistry 20 Chapter 2 Alkanes Review Carbon • Carbon has four valence electrons; hydrogen has one. • • C • H• • • To obtain an octet, carbon forms four bonds. H •• HC H •• H H H H C H H CH4 , methane Hydrocarbons Large family of organic compounds Composed of only carbon and hydrogen Saturated hydrocarbons Unsaturated hydrocarbons Alkanes H Alkenes, Alkynes & Aromatics H C-C C=C H C C C C C H CC C H Alkanes Methane Tetrahedral Expanded structural formula (Lewis): showing each bond line. Molecular formula CH4 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 Prefix + ane CnH2n+2 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 -yl • Halogen atoms are named as halo. -ine -O -OH -NO2 Hydroxyl Nitro 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 CH3─CH─CH2─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 CH3─CH─CH─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 1 2 3 4 2,4-dimethylpentane 5 Cl CH3 | | CH3─CH2─CH─CH2─C─CH2─CH3 | Cl 7 6 5 4 3 2 1 3,5-dichloro-3-methylheptane CH2 CH3 CH3─CH─CH2─CH3 STEP 1 Longest chain has 5 carbon atoms (Pentane). STEP 2 Number chain from end nearest substituent. 1 2 CH2 CH3 CH3─CH─CH2─CH3 3 STEP 3 4 5 Locate substituent and name. 3-Methylpentane Constitutional Isomers • Have the same molecular formula. • Have different atom arrangements (different structural formula). CH3CH2CH2CH3 Butane C4H10 2-Methylpropane C4H10 CH3 CH3CHCH3 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. OH Cl 2 CH3 1 Chlorocyclohexane Br 1-Bromo-2-methylcyclohexane 3 1 Cl 2 NO2 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 rotate by 120° 3 rotate by 60° 4 Least crow ded conformation Intermed iate crow din g Most crow d ed conformation 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. axis th rough the cen ter of th e rin g H H H H H H H H (a) Ball-and-s tick mod el sh ow ing all 12 hydrogen s H (b) The s ix eq uatorial C-H bond s Equatorial H H H (c) The s ix axial C-H bAxial on ds 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 CH3 CH3 Equatorial methylcyclohexane More Stable Axial methylcyclohexane Cis & Trans Stereoisomers The same molecular formula and the same connectivity of their atoms, but a different arrangement of their atoms in space. H H H H H H H H H H H H H3 C H CH3 CH3 cis-1,2-D imeth ylcyclop entane H H CH3 H trans -1,2-D imethylcyclop entane mp & bp of cis < mp & bp of trans Cis & Trans Stereoisomers CH3 H CH3 H CH3 H or H3 C H CH3 trans -1,4-D imethylcyclohexane or H3 C CH3 CH3 cis-1,4-D imethylcyclohexane 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 ↑ Number of branches ↑ CH3CH2CH2CH3 bp & mp ↑ bp & mp ↓ CH3 CH3CHCH3 Chemical reactions of Alkanes Low reactivity 1- Combustion: • Alkanes react with oxygen. • CO2, H2O, and energy are produced. • Alkane + O2 CH4 + 2O2 CO2 + H2O + heat CO2 + 2H2O + energy Chemical reactions of Alkanes Low reactivity 2- Halogenation: Alkanes react with Halogens. CH4 + Cl2 CH3Cl + HCl Chloromethane CH2Cl2 + HCl Dichloromethane CHCl3 + HCl Trichloromethane CCl4 + HCl Tetrachloromethane Heat or light CH3Cl+ Cl2 CH2Cl2 + Cl2 CHCl3 + Cl2 Heat or light Heat or light 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 2methylpropane. • Petroleum – A thick liquid mixture of thousands of compounds, most of them hydrocarbons, formed from the decomposition of marine plants and animals.