Bettelheim, Brown, Campbell and Farrell Chapter 11 Alkanes Bettelheim, Brown, Campbell and Farrell Chapter 11

Hybrid Orbitals Type Bond Angle Bonds to ? Atoms sp 180o 2 sp2 120o 3 Each hybrid orbital is identical

Condensed Structural Formula Molecular Formula: Gives number of each type of atom in compound Structural Formula Gives number of each type of atom AND shows how the atoms are connected Condensed Structural Formula Gives number of each type of atom AND shows how the atoms are connected using “shorthand” structures

Line-Angle Structural Formulas Line represents C-C single bond Each vertex (point) of angle represents a C atom Hydrogen atoms are not shown

Hydrocarbons Contain only carbon and hydrogen Saturated--only single bonds Unsaturated—not just single bonds

Alkanes Saturated hydrocarbons Contain only single bonds May be straight-chains, branched or cyclic First two alkanes are methane and ethane

Alkanes First 10 “straight-chain” alkanes No branches Generic Formula CnH2n+2 (n = number of C atoms)

ALKANES Straight chain or “normal” alkanes have all of their carbons connected in sequence C1 connected to C2 connected to C3 connected to C4, etc. A straight chain has angles of 109.5o between adjacent carbons. Physically the straight chain appears to be “zig-zag” rather than a straight line

Constitutional Isomerism (Structural Isomers) Compounds that have the same molecular formula but different structural formulas Up to 3 C—only one structure possible For 4 C (C4H10), two constitutional isomers are possible

Constitutional Isomerism Do the structural formulas in each set represent the same compound or constitutional isomers?

Free Rotation around C-C Single Bonds Conformations Different shapes which molecule can have when it rotates around single bonds

Constitutional Isomerism (a) The same compound—just different conformations (b) Constitutional isomers—connected differently

Constitutional Isomerism Draw structural formulas for the five constitutional isomers of molecular formula C6H14

Constitutional Isomerism Draw structural formulas for the five constitutional isomers of molecular formula C6H14

IUPAC Names for Unbranched Alkanes The IUPAC name has two parts: (1) a root name (prefix that shows the number of carbon atoms in the chain) (2) the suffix -ane: shows that the compound is a saturated hydrocarbon Root names for 1 to 4 carbons new Root names for 5 or more carbons same as molecular prefixes

Alkyl Group Names Substituent group derived from alkane by removing a hydrogen atom commonly represented by the symbol R- named by dropping the -ane from the name of the parent alkane and adding the suffix -yl

IUPAC Names The name of an alkane with a branched chain of carbon atom consists of: 1. Parent name: the longest chain of carbon atoms 2. Substituent names and locations: the groups bonded to the parent chain

IUPAC Alkane Names 1.An unbranched alkane: Prefix showing the number of carbon atoms plus -ane 2.Branched-chain alkanes: Parent name is name of longest carbon chain 3. Name and number each substituent on the parent chain: Use a hyphen between number and name

IUPAC Names 4.Number the parent chain from the end that gives the substituents the lower numbers

IUPAC Names 5. If the same substituent occurs more than once, Number parent chain from the end that gives the lower number to the substituent encountered first Use prefix (di-, tri-, tetra-,penta-, hexa- etc.) to show how many times the substituent Use a comma to separate numbers

IUPAC Names 6. For two or more different substituents List them in alphabetical order Number chain to give the lower number to the substituent encountered first For different substituents in same positions on opposite ends of the parent chain, the substituent that is first in the alphabet gets the lower number

IUPAC Names 7. Do NOT alphabetize the prefixes di-, tri-, tetra, etc., or the prefixes sec- and tert- in alphabetizing; Alphabetize the names of substituents first, and then insert these prefixes

Common Names Common names still in use Number of carbon atoms determines the name First three alkanes are methane, ethane, and propane All alkanes of formula C4H10 are butanes, all alkanes of formula C5H12 are called pentanes, etc. The prefix iso shows that one end of an otherwise unbranched chain terminates in (CH3)2CH- For more complex alkanes, use the IUPAC system

Name these compounds

Cycloalkanes Cyclic hydrocarbon: a hydrocarbon that contains carbon atoms joined to form a ring Cycloalkane: a cyclic hydrocarbon in which all carbons of the ring are saturated Cycloalkanes of ring sizes ranging from 3 to over 30 carbon atoms are found in nature Five-membered (cyclopentane) and six-membered (cyclohexane) rings are especially abundant in nature

Generic Formula Alkane: CnH2n+2 n = # of C Cycloalkane: CnH2n Note that making a ring results in 2 fewer hydrogens than in straight chain

Cycloalkanes Nomenclature Add cyclo- to the name of the corresponding open-chain alkane and name each substituent on the ring If only one substituent, it does not need to be numbered If there are two substituents, number the ring beginning with the substituent of lower alphabetical order.

Conformations - Alkanes Conformation: any three-dimensional arrangement of atoms in a molecule that results by rotation about a C-C single bond Three conformations for a butane molecule

Conformations - Alkanes Ring formation limits free rotation Can only rotate partly without breaking bonds

Cyclopentane The most stable conformation of a cyclopentane ring is an envelope conformation

Cyclohexane The most stable conformation of a cyclohexane ring is the chair conformation all bond angles are approximately 109.5°

Cyclohexane

Cyclohexane In a chair conformation, six C-H bonds are equatorial (red) six C-H bonds are axial (blue)

Cyclohexane the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

Cis-Trans Isomers Cis: on the same side of ring Trans: on opposite sides of ring Look at molecule edge-on… Same side Opposite side

Cis-Trans Isomers ….or view it from above (use wedge notation)

Cis-Trans Isomers Often helpful to picture cyclohexane ring as hexagon to determine which cis-trans isomer is present Stereoisomers differ in the orientation of atoms in space (connected to same atoms) Cis-trans isomers are one type of stereoisomers

Physical Properties of Alkanes Most important physical property of alkanes and cycloalkanes is their almost complete lack of polarity Electronegativity difference between carbon and hydrogen is 2.5 - 2.1 = 0.4 Thus C-H bond is nonpolar covalent Alkanes are nonpolar compounds Display only weak London dispersion forces between molecules

Physical Properties of Alkanes Melting and boiling points Low melting and boiling points compared to other types of compounds with same size Both boiling and melting points of alkanes increase with increasing molecular weight Greater number of London dispersion forces

Physical Properties of Alkanes

Physical Properties of Alkanes Constitutional isomers are different compounds with different physical and chemical properties C6H14 compounds:

Physical Properties of Alkanes Solubility: “like dissolves like” Nonpolar alkanes are NOT soluble in water Do not form hydrogen bonds with water Soluble in each other and in other nonpolar organic compounds Density Densities of 0.7-0.8 g/mL Less dense than water (1.0 g/mL) so they float on water

Chemical Properties of Alkanes Oxidation (Combustion) Reaction with oxygen Products are CO2, H2O and heat Used as energy sources for heat and power

Alkyl Halides Alkanes with halide substituents F Cl Br I CH2Cl2 CH3Cl CHCl3 CCl3F (Freon-11)

Reactions Reaction with halogens (halogenation) Substitution reaction Requires heat or light as catalyst

The Chlorofluorocarbons Chlorofluorocarbons (CFCs) Freons CCl3F (Freon-11) and CCl2F2 (Freon-12) nontoxic, nonflammable, odorless, noncorrosive CFCs were used as refrigerants industrial cleaning solvents propellants for aerosol sprays

CFC Replacements Chlorofluorocarbons (CFCs) cause destruction of the Earth’s stratospheric ozone layer Replacements are hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) More reactive than CFCs Destroyed before they reach the stratosphere

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

Refining of Crude Oil

Petroleum Distillation Fraction Size Distills at oC Gasoline C4 to C12 20 - 200 Kerosene C10 to C14 200 - 275 Fuel Oil/Diesel C14 to C18 275 - 350 Lubricating Oil C16 to C20 > 350 Residue >C20 (Asphalt/Greases)