ALKANES C n H 2n+2. Alkanes or C n H 2n+2 C 3 H 8 H H H H–C–C–C–H H H H CH 3 CH 2 CH 3...
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Transcript of ALKANES C n H 2n+2. Alkanes or C n H 2n+2 C 3 H 8 H H H H–C–C–C–H H H H CH 3 CH 2 CH 3...
Alkanes or CnH2n+2
C3H8
H H H
H–C–C–C–H
H H H
CH3CH2CH3
Chemical Formula
Structural Formula
Condensed Structural Formula
Alkanes: base unit
CH4
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 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
A. Low melting point
B. High melting point
C. Soluble in polar solvents
D. 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 straight-chain Alkanes
• Name describes molecule so can draw it
• All alkaneanes have the suffix –aneane
• prefix tells how many C’s
C5H12
C4H10
C3H8
C2H6
Meth + aneMeth + aneCH4
NameFormula
EthaneEthane
PropanePropane
ButaneButane
PentanePentane
Branched-chain alkanesBranched-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
IsomersIsomers
• 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 Different structures, different propertiesproperties
• 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!
Naming branched-chain alkanes• find longest continuous chainlongest continuous chain (backbone) of C
atoms (Bends don’t count!)• base namebase name derived from # of C’s in longest chain• branchesbranches named inin prefixprefix
– Branches also named by number of C’s – “branchbranch” part of name ends in “yl”ends in “yl”
• location of branch shown by assignassigning #’s to C’s #’s to C’s in backbonein backbone
• number from end that gives lowest # for branch• if more than 1 of same type branch:
– use di, tri, tetra
H
H–C–H
H H
H–C–C–C–H
H H H
Branch: 1 carbon – methylmethyl
Branch at C #2:- can’t be on any other C so doesn’t need #
Methyl propaneMethyl propane
C4H10
CH3CH(CH3)CH3
Longest continuous chain has 3 carbon atoms = propanepropane
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
2-methyl hexane2-methyl hexane
C7H16
CH3CH(CH3)CH2CH2CH2CH3
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 #