Chapter 19 - Organic Chemistry

19.2 Alkanes

hydrocarbons: compounds composed only of carbon and hydrogen atoms

alkanes: hydrocarbons whose molecules contain only single bonds

• saturated hydrocarbons: maximum ratio of hydrogen to carbon atoms

• unsaturated hydrocarbons: lower hydrogen-carbon ratios

kekule strucutres: structures of organic molecules that show all bonds with lines but omit any lone pairs

• carbon-skeleton structure: no letters used for carbon and hydrogen, each line segment represents one carbon-carbon bond with 109.5°. Each end of the zigzag line is a -CH3 and every intersection of two line segments is a -CH2- group. Assumed each carbon has a steric number of 4.

Alkane properties

• low reactivity (paraffins)

• widely used as fuels and lubricants; highly exothermic with a significant activation energy barrier

• homologous series: series of compounds win which members can be described by a general formula and similar chemical properties

  • linear straight-chain hydrocarbons: general formula of CnH2n+2

    • methylene group: -CH2- unit

    • methyl group: -CH3 unit

    • branch: side chain attached to the longest chain

  • structural (constitutional) isomers: compounds with same molecular formula but atoms connected in different bonding patterns

Naming alkanes

1. select longest chain of carbon atoms and use prefix given in table to name this as the parent chain ex. pentane

2. identify each branch and name it with prefix that matches the number of carbon atoms in the branch, append with suffix -yl; name of branch comes before the name of the parent chain ex. methylpentane

3. number carbon atoms in parent chain, place branches on lowest possible number starting on the left ex. 2-methylpentane

4. if the same group is attached more than once to the parent chain, we use prefixes di-, tri- and tetra- to indivate number of groups present. ex. 2,4-dimethylpentane

5. if different groups are attached to a parent chain, they are named in alphabetical order. ex. 4-ethyl-3-methylheptane (not considering prefixes)

Calcoalkanes: alkanes formed in a ring structure, general formula of CnH2n.

• Has one more carbon-carbon bond and two fewer hydrogen atoms per molecule than an alkane with same number of carbon atoms, bond angle of 109.5

• chair form: two possible configurations of cyclohexane with greatest structural stability; cyclohexane spends 99% of its time in one chair form or the other

  • boat conformation: higher-energy transition-state configurations (repulsion between the two hydrogen atoms across the ring from each other adds to internal energy and reduces stability) required to flip between chair conformations

    • axial position: hydrogen atoms above and below the carbon ring

    • equatorial position: hydrogen atoms more parallel to the ring

19.3 Alkenes and Alkynes

alkenes: hydrocarbons with one or more carbon-carbon double bonds

• one c-c double bond has one degree of unsaturation

• capacity to be reduced allows alkenes and alkynes to be more reactive than alkanes; among the most versatile functional groups in organic chemistry

  • electron density is greatest around the bonding axis, making pi electrons more accessible to reactions than the sigma bonds between carbon atoms, explaining why unsaturated hydrocarbons are more reactive

alkynes: hydrocarbons with one or more carbon-carbon triple bonds

• one c-c triple bond has two degrees of unsaturation

hydrogenation: alkenes and alkynes combine with H2 to form alkanes; each degree of unsaturation requires one H2 ex. 2 degrees requires 2H2

Z (cis) isomer: methyl group and chain after the double bond are on the same side of the structure

E (trans) isomer: methyl group and chain after the double bond are on opposite sides of the structure

• these isomers exist because there is no free rotation around the bond, both are stereoisomers

Naming Alkenes and Alkynes

• prefixes used to identify length of chain, suffix -ene for alkene and -yne for alkyne

• carbon atoms in chain are numbered so the first carbon in the double/tripple bond has the lowest number possible and it precedes the name with a hyphen

• stereoisomers are identified by writing cis- or trans- before the number ex. cis-2-pentene or trans-2-pentene

physical properties of monomer ethylene:

• Homopolymer: composed of one type of monomer

• Addition polymer: a polymer constructed by adding many molecules together to form the polymer chain

• vinyl polymer: CH2--CH-, where -- is a double bond

• Branched chains have lower densities and weaker intermolecular forces, causing them to be deormable and softer

• replacing hydrogens will change the chemical composition,

19.5 Amines

Amines: nitrogen is the defining component of functional group, organic base

• Primary amine: one hydrogen in NH3 replaced with an R group, RNH2

• Secondary amine: two hydrogens in NH3 replaced with an R group, R2NH

• Tertiary amine: three hydrogens in NH3 replaced with an R group, R3N

Heteroatoms: atoms other than carbon, hydrogen or metals present in organic compounds

19.6 Alcohols, Ethers and Reformulated Gasoline

Alcohols: general formula of R-OH where R is any alkyl group

• small R group behaves like water and a large R group acts more like a hydrocarbon

• Methanol: widely used industrial organic chemical, starting material in preparation of organic compounds used to make polymers

• Ethanol: formed from fermentation of sugar from vegetable sources; added to gasoline to promote complete combustion and reduce air pollution

Ethers: general formula of R-O-R where R is any alkyl group or aromatic ring

• polar molecules with water solubilities comparable to alcohols of similar molar mass but lower boiling points - more similar to alkanes of similar molar mass

Polymers of Alcohols and Ethers

• used to make adhesives, emulsions and less porous, absorbent and smoother materials

• -OH chains make surface polar and resistant to organic solvents

• Copolymer: two different monomers

• Heteropolymer: three or more different monomers

  • monomers forming these two types of molecules can form alternating, block and random co/heteropolymers

19.8 A Brief Survey of Isomers

Chain Isomers: molecules having different arrangements of their carbon skeletons

Positional isomers: same functional group (-OH group) bonded to different carbon atoms

Functional Isomers: different functional groups because of atom arrangement