ALKANES CnH2n+2

Alkanes or CnH2n+2 C3H8 H H H H–C–C–C–H CH3CH2CH3 Chemical Formula Structural Formula Condensed Structural Formula

Alkanes: base unit CH4 CH3CH3 or C2H6 CH3CH2CH3 or C3H8 CH3CH2CH2CH2CH3 or C5H12 Difference between each is CH2

Properties of Alkanes Change systematically with number of C’s As number of C’s increases, boiling point ↑ molecules get heavier & it’s harder to change into gas phase

Properties of Alkanes Low Reactivity Except readily undergo combustion – fuels Nonpolar – Don’t dissolve well in water Low mp’s & bp’s mp & bp ↑ with ↑ molecular mass High vapor pressures

Which property is generally characteristic of an organic compound? Low melting point High melting point Soluble in polar solvents Insoluble in nonpolar solvents Correct response = A

Which of the following compounds has the highest boiling point? Correct answer = D bp depends on strength of forces holding molecules together [forces ↑as size of molecule ↑]

Naming Hydrocarbons

Naming straight-chain Alkanes Name describes molecule so can draw it All alkanes have the suffix –ane prefix tells how many C’s

10 Dec 9 Non 8 Oct 7 Hept 6 Hex 5 Pent 4 But 3 Prop 2 Eth 1 Meth # of C atoms Prefix

C5H12 C4H10 C3H8 C2H6 Meth + ane CH4 Name Formula Ethane Propane Butane Pentane

Branched-chain alkanes Beginning with butane, C4H10, more than 1 way to arrange the atoms H H–C–H H H H–C–C–C–H H H H

Branched alkane - can’t link all C’s without lifting pencil off paper Methyl propane

Isomers Compounds with same molecular formula but different structural arrangement more C atoms in the formula, the more isomers  more possible ways to arrange them

Isomers Different structures, different properties Isomers have different chemical and physical properties

A note about isomers --- If comparing 2 structural formulas & you can superimpose them, then not isomers = same molecule! If can rotate or flip one of structural formulas & superimpose it on the other, they are not isomers – they are the same molecule!

These diagrams represent the same molecule Represents bond to H

These diagrams represent the same molecule

These diagrams represent the same molecule

Two different molecules!

Branched Hydrocarbons Naming Branched Hydrocarbons

Naming Branched-Chain Alkanes goal of name: to describe molecule so can draw it

Naming branched-chain alkanes find longest continuous chain (backbone) of C atoms (Bends don’t count!) base name derived from # of C’s in longest chain branches named in prefix Branches also named by number of C’s “branch” part of name ends in “yl” location of branch shown by assigning #’s to C’s in backbone number from end that gives lowest # for branch if more than 1 of same type branch: use di, tri, tetra

Branch: 1 carbon – methyl Branch at C #2: - can’t be on any other C so doesn’t need # Methyl propane C4H10 CH3CH(CH3)CH3 H H–C–H H H H–C–C–C–H H H H Longest continuous chain has 3 carbon atoms = propane

Longest continuous chain: 6 C’s = hexane branch is 1 carbon long – methyl. branch located at C #2 2-methyl hexane C7H16 CH3CH(CH3)CH2CH2CH2CH3 H H–C–H H H H H–C–C–C–C–H H H H

A note about branches If have several branches with same # C’s can condense name a bit 2-methyl 3-methyl pentane becomes 2,3-dimethyl pentane every branch must have a #

Longest continuous chain has 6 C’s: hexane -branch 1 C = methyl -branch located at C #3 3-methyl hexane C7H16 CH3CH2CH2CH(CH3)CH2CH3 CH3(CH2)2CH(CH3)CH2CH3 H H – C – H H H H H – C – C – C – C – H H H H–C–H H–C–H H H