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11
-Structural isomers-Naming Branched Alkanes
Mr. Shields Regents Chemistry U16 L02
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IsomersIsomers
Compounds with the Compounds with the same molecular formulasame molecular formula but different structural arrangement are but different structural arrangement are known as structural isomers.known as structural isomers.
The more C atoms there are in the formula, The more C atoms there are in the formula, the more structural isomers there will be. the more structural isomers there will be.
4C (1) 5C (3) 6C (5) 8C (18)
20 C (>303,000)
33
Structural IsomersStructural Isomers In order to determine if 2 In order to determine if 2
molecules are isomers of molecules are isomers of one another first determine one another first determine the molecular formula’s of the molecular formula’s of bothboth
To the right we haveTo the right we have pentane (Top)pentane (Top) 2 – methyl butane (bottom)2 – methyl butane (bottom)
What is the mol. Formula What is the mol. Formula for each? Are theyfor each? Are they
structural isomers?structural isomers?
C5H12 – Yes, theyAre isomers
44
IsomersIsomers
Isomers have Isomers have differentdifferent structuresstructures and and differentdifferent chemicalchemical and and physicalphysical properties. properties.
Butane 2-methylpropaneFormula C4H10 C4H10
B.P. 0 C -12 CM.P. -138 C -159 CDensity 0.622 0.604Sol. In 100ml Alcohol 1813 ml 1320 ml
CH3CH2OH and CH3OCH3 are also isomers of one anotherNOTE!
55
Recall that the members of the group of alkanesForms a homologous series and each member ofThis series differs from the last by 1 –CH2- unit
When we draw the structural formulas of the 1st
Three members of this group there is only oneWay each can be drawn.
CH4 CH3-CH3 CH3-CH2-CH3
Drawing simple alkanesDrawing simple alkanes
66
Branched-chain alkanesBranched-chain alkanes
Beginning with Beginning with butanebutane, , CC44HH1010, there is , there is more than 1 more than 1 wayway to arrange the atoms besides one carbon after to arrange the atoms besides one carbon after another (i.e. straight chain).another (i.e. straight chain).
Can you figure out how many ways it can be drawn?Can you figure out how many ways it can be drawn?
CH3-CH2-CH2-CH3 & CH3
|CH3-CH-CH3
77
Branched-chain alkanesBranched-chain alkanes In both butane structures we have the same In both butane structures we have the same
numbers and kind of atoms namely, numbers and kind of atoms namely, CC44HH1010
The general formula for each is also The general formula for each is also CCnnHH2n+22n+2 so each so each represents the alkane “Butane”represents the alkane “Butane”
Yet there is a difference. Yet there is a difference. The difference lies in what The difference lies in what atoms are joined to what atomsatoms are joined to what atoms
CH3-CH2-CH2-CH3 CH3
* |CH3-CH-CH3
*
C4H10 C4H10
Note that these are totally different compounds having there own unique Chemical & Physical Properties!
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Note that what we have drawn for butane representsReal structural changes and not just apparent changesResulting from the rotation of a C-C single bond.
Let’s Look a pentane to see what we mean by this.
Rotation of this bondLeads to this configurationOf pentane
And rotation of thisBond leads to this
All of these structures represent the SAME molecule!
99
Pentane isomersPentane isomersTo find new structural isomers of the straight chain Alkanes we need to move the point of attachment ofThe various carbon atoms.
Remember, carbonMust have 4 bonds.No more, No less!
So how many structuralIsomers does pentaneHave?
Straight chainisomer
1010
Naming branched-chain Naming branched-chain alkanesalkanes
Find the longest continuous chain or backbone of C atoms. Find the longest continuous chain or backbone of C atoms.
c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-ccc c c What’s the longest chain?What’s the longest chain?
cc
The base name is derived from the number of C’s in the longest chain.The base name is derived from the number of C’s in the longest chain.
10 carbons would be decane
1111
Naming branched-chain Naming branched-chain alkanesalkanes
Branches are added Branches are added as a prefix and are as a prefix and are named by counting named by counting the number of C the number of C atoms.atoms.
The “branch” alkane The “branch” alkane name ends in “yl.”name ends in “yl.”
MethylMethyl CHCH33——
EthylEthyl CHCH33CHCH22 – –
PropylPropyl CHCH33CHCH22CHCH22 ––
ButylButyl CHCH33CHCH22CHCH22
CHCH22 – –
c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-c c cc c c c
1212
Naming branched-chain Naming branched-chain alkanesalkanes
The location of the branch (or substituent group) is shown The location of the branch (or substituent group) is shown by assigning numbers to the C’s in the backbone. by assigning numbers to the C’s in the backbone.
Number from the end that gives the lowest number for Number from the end that gives the lowest number for the first branch.the first branch. Substituent groups (branches) are listed alphabeticallySubstituent groups (branches) are listed alphabetically
There may be more than 1 of the same type of branch. There may be more than 1 of the same type of branch. Use di, tri, tetra etc. for 2, 3, and 4 Use di, tri, tetra etc. for 2, 3, and 4 Number the locations and separate the nos. by a comma &Number the locations and separate the nos. by a comma & separate the last no. from the name by a dash
7-ethyl-3-methyldecane
c-c-c-c-c-c-c-c-c-cc-c-c-c-c-c-c-c-c-c c cc c c c
1 2 3 4 5 6 7 8 9 10
1313
Longest continuous chain has 4carbon atoms – butane.
2 Branches each have 1 carbon – dimethyl.Branches are at C-2. WHY??
2,2-Dimethylbutane or
C6H14 or CH3C(CH3)2CH2CH3
CH3 | H3C-C-CH2-CH3
| CH3
Name this compound
1414
H
H–C–H
H H
H–C–C–C–H
H H
H-C-H|
H
Longest continuous chain has 3carbon atoms – propane.
2 Branches each have 1 carbon – dimethyl. And … bothBranches have to be at C-2. WHY??
So ….Only specify the branch number if necessary.
Ex: this is Dimethylpropane & not
2,2-dimethylpropane
Example:
1515
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 carbon atoms: hexaneBranch is 1 carbon long: methylBranch is located at C-3
3-methylhexane or
C7H16 or
CH3CH2CH2CH(CH3)CH2CH3
Name this compound:
1616
More Naming ProblemsMore Naming Problems
CH
H3C CH
CH2
CH2
CH2
CH
CH3
CH3
CH3 CH3
CH
H3C CH2
CH3
CH3Methylbutane
2,3,7-Trimethyloctane
Dimethylpropane
3-ethylpentane
CH3CH(CH3)CH(CH3)CH2CH2CH2CH(CH3)CH3
1717
More Naming ProblemsMore Naming Problems
C C C | | |C-C-C-C-C-C-C | C |
C
What is the nameOf this compound ?
4-ethyl-2,5-dimethyloctane
WOW !
1818
Problems:Problems: Draw the structural Draw the structural
formula for the formula for the following:following:
methylpropanemethylpropane 3 – ethyl – 4 - 3 – ethyl – 4 -
methylnonanemethylnonane dimethylpropanedimethylpropane 2,3,4 – 2,3,4 –
trimethyldecanetrimethyldecane
What name does this compound have?3-ethyl-3-methylpentane
1919
Name this compoundName this compound
C-C-C | C-C-C-C-C-C-C-C | | | C C C | C
6,6-diethyl-3,5-dimethylnonane
1
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