ALKANES C n H 2n+2
Jan 18, 2018
ALKANES
CnH2n+2
Alkanes or CnH2n+2
C3H8
H H HH–C–C–C–H
H H H
CH3CH2CH3
Chemical Formula
Structural Formula
Condensed Structural Formula
Alkanes: base unitCH4
CH3CH3 or C2H6
CH3CH2CH3 or C3H8
CH3CH2CH2CH3 or C4H10
CH3CH2CH2CH2CH3 or C5H12
Difference between each is CHCH22
Properties of Alkanes
• Change systematically with number of C’s
• As number C’s increases, boiling point ↑– molecules get heavier
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
– evaporate easily
A. Low melting pointB. High melting pointC. Soluble in polar solventsD. Insoluble in nonpolar solvents
Which property is generally characteristic of an organic compound?
Correct response = ACorrect response = A
A. CH4
B. C2H6
C. C3H8
D. C4H10
Which of the following compounds has the highest boiling point?
Correct answer = D Correct answer = D
bp depends on strength of forces holding molecules bp depends on strength of forces holding molecules together [forces together [forces ↑↑as size of molecule as size of molecule ↑]
Naming Hydrocarbons
Naming straight-chain Alkanes
• Name describes molecule so can draw it
• All alkaneanes have the suffix –aneane• prefix tells how many C’s
10Dec9Non8Oct7Hept6Hex5Pent4But3Prop2Eth1Meth
# of C atomsPrefix
C5H12
C4H10
C3H8
C2H6
Meth + aneMeth + aneCH4
NameFormula
EthaneEthane
PropanePropaneButaneButanePentanePentane
Branched-chain alkanesBranched-chain alkanes
Beginning with butane, C4H10, more than 1 way to arrange the atoms
HH–C–HH H
H–C–C–C–HH H H
Branched alkane- can’t link all C’s without lifting pencil off paper
Methyl propane
IsomersIsomers
• Compounds with same molecular formula but different structural arrangement
• more C atoms more isomers (more possible ways to arrange them)
Isomers
• Different structures = different Different structures = different propertiesproperties–different chemical & physical properties
A note about isomers ---
• comparing 2 structural formulas: if 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 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!
Naming Branched
Hydrocarbons
Naming Branched-Chain Alkanes
• goal of name: describe molecule so can draw it
Naming branched-chain alkanes• find longest continuous chainlongest continuous chain (backbone) of C
atoms (Bends don’t count!)• base namebase name derived from # C’s in longest chain• branchesbranches named firstfirst
– Branches named by # C’s – “branchbranch” name ends in “yl”ends in “yl”
• assignassign #’s to C’s in backbone#’s to C’s in backbone • # from end so lowest # given to branch• more than 1 same type branch:
– use di, tri, tetra
HH–C–HH H
H–C–C–C–HH H H
Branch: 1 carbon – methylmethyl
Branch at C #2:- can’t be on any other C so doesn’t need #
Longest continuous chain has 3 carbon atoms = propanepropane
11 22 33
Methyl propaneMethyl propane
C4H10
CH3CH(CH3)CH3
HH–C–H
H–C–H H H HH–C–C–C–C–H H H H H–C–H H
Longest continuous chain: 6 C’s = hexanehexane
branch is 1 carbon long – methyl methyl.
branch located at C #22
C7H16
CH3CH(CH3)CH2CH2CH2CH3
33 11
66
55
4422
2-methyl hexane2-methyl hexane
A note about branches
• If 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 #
H
H – C – H
H H HH – C – C – C – C – H H H H–C–H H–C–H H H
Longest continuous chain has 6 C’s: hexane
-branch 1 C = methyl-branch located at C #3
C7H16
CH3CH2CH(CH3)CH2CH2CH3
3-methyl hexane