3 ch241 alkanes (2)
TRANSCRIPT
3. Organic Compounds: Alkanes and 3. Organic Compounds: Alkanes and Their StereochemistryTheir Stereochemistry
Why this ChapterWhy this ChapterAlkanes are unreactive, but provide useful vehicle to introduce important ideas about organic compounds
Alkanes will be used to discuss basic approaches to naming organic compounds
We will take an initial look at 3-D aspects of molecules
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3.1 Functional Groups3.1 Functional Groups Functional group -
collection of atoms at a site that have a characteristic behavior in all molecules where it occurs
The group reacts in a typical way, generally independent of the rest of the molecule
For example, the double bonds in simple and complex alkenes react with bromine in the same way
Functional Groups with: Functional Groups with: Multiple Carbon–Carbon BondsMultiple Carbon–Carbon Bonds
Alkenes have a C-C double bondAlkynes have a C-C triple bond
Arenes have special bonds that are represented as alternating single and double C-C bonds in a six-membered ring
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Alkenes and AlkynesAlkenes and Alkynes
Alkenes contain a double bond between adjacent carbon atoms.
Alkynes contain a triple bond.
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Functional Groups with: Functional Groups with: Carbon Singly Bonded to an Electronegative AtomCarbon Singly Bonded to an Electronegative Atom
Alcohols and EthersAlcohols and EthersAn alcohol contains
the hydroxyl (-OH) functional group.
In an ether, an oxygen atom is bonded to two carbon atoms –C–O–C– .
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Functional Groups with: Functional Groups with: Carbon–Oxygen Double Bond (Carbonyl Groups)Carbon–Oxygen Double Bond (Carbonyl Groups)
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Aldehydes and KetonesAldehydes and KetonesAn aldehyde contains a
carbonyl group (C=O), which is a carbon atom with a double bond to an oxygen atom, attached to at least one hydrogen.
In a ketone, the carbon of the carbonyl group is attached to two other carbon atoms.
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Carboxylic Acids and EstersCarboxylic Acids and Esters
Carboxylic acids contain the carboxyl group, which is a carbonyl group attached to a hydroxyl group. O
— C—OHAn ester contains the carboxyl
group between carbon atoms.
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Amines and AmidesAmines and Amides
In amines, the functional group is a nitrogen atom. |
—N —
In amides, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group.
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Summary of Functional GroupsSummary of Functional Groups
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Survey of Functional GroupsSurvey of Functional Groups
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Survey of Functional GroupsSurvey of Functional Groups
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Methyl amine (fish)
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Functional Groups in Everyday Functional Groups in Everyday ItemsItems
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Learning CheckClassify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine.
1) CH3–CH2–CH2–OH
2) CH3–O–CH2–CH3
3) CH3–CH2–NH2
O O || ||
4) CH3–C–OH 5) CH3–C–O–CH3
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SolutionClassify each of the following as alcohol, ether, aldehyde, ketone, carboxylic acid, ester, or amine.1) CH3─CH2─CH2─OH 2) CH3─O─CH2─CH3
alcohol ether
3) CH3─CH2─NH2 amine O O || ||
4) CH3─C─OH 5) CH3─C─O─CH3
carboxylic acid ester
What functional group is not present in the naturally What functional group is not present in the naturally occurring compound shown below? occurring compound shown below?
1. carboxylic acid2. amine3. primary alcohol4. secondary alcohol5. methyl group
H3C OH
OOH
NH2
Learning Check
What functional group is not present in the naturally What functional group is not present in the naturally occurring compound shown below? occurring compound shown below?
1. carboxylic acid2. amine3. primary alcohol4. secondary alcohol5. methyl group
H3C OH
OOH
NH2
Solution
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3.2 Alkanes and Alkane Isomers3.2 Alkanes and Alkane IsomersAlkanes: Compounds with C-C single bonds and C-H bonds
only (no functional groups)Connecting carbons can lead to large or small moleculesThe formula for an alkane with no rings in it must be
CCnnHH2n+22n+2 where the number of C’s is nAlkanes are saturatedsaturated with hydrogen (no more can be
addedThey are also called aliphatic compounds
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Condensed Structures of AlkanesCondensed Structures of AlkanesWe can represent an alkane in a brief form or in many
types of extended formA condensed structure does not show bonds but lists
atoms, such as ◦ CH3CH2CH2CH3 (butane)
◦ CH3(CH2)2CH3 (butane)
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Expanded and Condensed StructuresExpanded and Condensed Structures
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Some Structures for ButaneSome Structures for Butane
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Line-Bond FormulasLine-Bond FormulasBecause each C atom has a tetrahedral
arrangement, the order of atoms is not a straight line, but a zigzag pattern.
A line-bond formula abbreviates the carbon atoms and shows only the zigzag pattern of bonds from carbon atom to carbon atom.
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Hexane has Six Carbon AtomsHexane has Six Carbon Atoms
Hexane Is an alkane with 6 carbon atoms in a
continuous chain.Has a “zig-zag” look because each carbon
atom is at the center of a tetrahedron.Is represented by a ball-and-stick model
as
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Learning Check Learning Check A. Write the condensed formula for:
H H H H H
H C C C C C H
H H H H H
B. What is its molecular formula?
C. What is its name?
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SolutionSolutionA. Write the condensed formula for:
H H H H H
H C C C C C H
H H H H H
B. What is its molecular formula?
C. What is its name?
CH3CH2CH2CH2CH3
C5H12
pentane
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Learning CheckLearning CheckWrite the condensed structural formula for
A. ethane
B. heptane
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SolutionSolutionWrite the condensed structural formula for
A. ethane CH3 CH─ 3
B. heptane CH3 CH─ 2 CH─ 2 CH─ 2 CH─ 2 CH─ 2 CH─ 3
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Alkane IsomersAlkane IsomersCH4 = methane, C2H6 = ethane, C3H8= propane
The molecular formula of an alkane with more than three carbons can give more than one structure◦ C4 (butane) = butane and isobutane
◦ C5 (pentane) = pentane, 2-methylbutane, and 2,2-dimethylpropane
Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes
Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes
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Constitutional IsomersConstitutional Isomers Isomers that differ in how their atoms are arranged in
chains are called constitutional isomersCompounds other than alkanes can be constitutional
isomers of one anotherThey must have the same molecular formula to be isomers
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Isomers of ButaneIsomers of ButaneIsomersHave the same
molecular formula.
Have different atom arrangements.
Of butane (C4H10) consist of a straight chain and a branched chain each with 4 carbon atoms.
branch
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What kind of carbon What kind of carbon is notis not present in the molecule present in the molecule below?below?
1. primary2. secondary3. tertiary4. quaternary5. All 4 kinds of carbon
are present
HO
Learning Check
What kind of carbon What kind of carbon is notis not present in the molecule present in the molecule below?below?
1. primary2. secondary3. tertiary4. quaternary5. All 4 kinds of carbon
are present
HO
Solution
How many secondary hydrogens are there in the How many secondary hydrogens are there in the following molecule?following molecule?
1. 22. 33. 44. 55. 6
Learning Check
How many secondary hydrogens are there in the How many secondary hydrogens are there in the following molecule?following molecule?
1. 22. 33. 44. 55. 6
Solution
Which of the following choices Which of the following choices is notis not an an alkane?alkane?
1. C12H26
2. C10H20
3. C11H24
4. C16H34
5. C20H42
Learning Check
Which of the following choices Which of the following choices is notis not an an alkane?alkane?
1. C12H26
2. C10H20
3. C11H24
4. C16H34
5. C20H42
Solution
A brilliant chemist determined the number of A brilliant chemist determined the number of secondary secondary carbons and carbons and secondarysecondary hydrogens for an unknown alkane. hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for numbers are correct. Which of these answers is possible for an alkane?an alkane?
2° hydrogens 2° carbons
1. 6 22. 4 43. 3 54. 3 65. 8 4
Learning Check
A brilliant chemist determined the number of A brilliant chemist determined the number of secondary secondary carbons and carbons and secondarysecondary hydrogens for an unknown alkane. hydrogens for an unknown alkane. Well, actually, the chemist is having trouble deciding which Well, actually, the chemist is having trouble deciding which numbers are correct. Which of these answers is possible for numbers are correct. Which of these answers is possible for an alkane?an alkane?
2° hydrogens 2° carbons
1. 6 22. 4 43. 3 54. 3 65. 8 4
Solution
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3.4 Naming Alkanes3.4 Naming AlkanesCompounds are given systematic names by a process that
uses
Follows specific rules◦ Find parent hydrocarbon chain◦ Carbons in that main chain are numbered in sequence◦ Substituents are identified numbered◦ Write compound name is single word ◦ Name a complex substituents as though it were a
compound itselfSee specific examples in text
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IUPAC Names for Simple AlkanesIUPAC Names for Simple Alkanes•Use a prefix to indicate the number of carbons in a chain.•End in –ane.
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Learning CheckLearning CheckA. Give the name of each compound:
1) CH3—CH3
2) CH3—CH2—CH3
3) CH3—CH2—CH2—CH2—CH2—CH3
B. Write the condensed structural formula of pentane.
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SolutionSolutionA. Give the name of each compound:
1) CH3—CH3 ethane
2) CH3—CH2—CH3 propane
3) CH3—CH2—CH2—CH2—CH2—CH3
hexaneB. Write the condensed structural formula of
pentane.CH3—CH2—CH2—CH2—CH3
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3.3 Alkyl Groups3.3 Alkyl GroupsAlkyl group – remove one H from an alkane (a part of a
structure)
General abbreviation “R” (for Radical, an incomplete species or the “rest” of the molecule)
Name: replace -ane ending of alkane with -yl ending◦ CH3 is “methyl” (from methane)
◦ CH2CH3 is “ethyl” from ethane
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Types of Alkyl groupsTypes of Alkyl groups
Classified by the connection site (See Figure 3.3)◦ a carbon at the end of a chain (primary alkyl group)◦ a carbon in the middle of a chain (secondary alkyl group)◦ a carbon with three carbons attached to it (tertiary alkyl
group)
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Naming SubstituentsNaming SubstituentsCH3
CH2 CH3
CH2 CH2 CH3
CHCH3
CH3
CH2 CH
CH3
CH3
CH2 CH2 CH2 CH3
CH
CH3
CH2 CH3
C
CH3
CH3
CH3
IUPAC Common
methyl methyl
ethyl ethyl
propyl n-propyl
1-methylethyl Isopropyl
butyl n-butyl
1-methylpropyl sec-butyl
2-methylpropyl Isobutyl
1,1-dimethylethyl tert-butyl ort-butyl
IUPAC Common
pentyl n-pentyl oramyl
1-methylbutyl sec-pentyl
3-methylbutylisopentyl orisoamyl or i-amyl
1,1-dimethylpropyl
tert-pentyl ortert-amyl or t-amyl
2,2-dimethylpropyl
neopentyl
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Naming SubstituentsNaming Substituents
CH2 CH2 CH CH3
CH3
CH2 CH2 CH2 CH2
CH
CH3
CH2 CH2
C
CH3
CH2
CH3
CH3
CH3
CH3
CH2 C
CH3
CH3
CH3
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Naming SubstituentsNaming SubstituentsIUPAC
Fluoro
Chloro
Bromo
Iodo
Hydroxy
F
Cl
Br
I
OH
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Alkanes with Alkyl GroupsAlkanes with Alkyl Groups
CH3 CH CH3
CH3 methylpropane
2,4-dimethylpentane
methyl groupsmethyl groups
CH3 CH
CH3
CH2 CH
CH3
CH3
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Write three isomers of C5H12 and name each.
Learning CheckLearning Check
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Write three isomers of C5H12 and name each.
SolutionSolution
A chain of 5 carbon atoms CH3 CH─ 2 CH─ 2 CH─ 2 CH─ 3
pentane
A chain of 4 carbon atoms and 1 carbon branch CH3
│CH3 CH CH─ ─ 2 CH─ 3 2-methylbutane
A chain of 3 carbon atoms and 2 carbon branches CH3
│CH3 C CH─ ─ 3 2,2-dimethylpropane │ CH3
What is the best IUPAC name for the following What is the best IUPAC name for the following compound?compound?
1. 2-ethyl-3-isopropylhexane2. 1-isopropyl-3-ethylhexane3. 4-isopropyl-5-ethylhexane4. 3-methyl-4-isopropylheptane5. 4-isopropyl-3-methylheptane
Learning CheckLearning Check
What is the best IUPAC name for the following What is the best IUPAC name for the following compound?compound?
1. 2-ethyl-3-isopropylhexane2. 1-isopropyl-3-ethylhexane3. 4-isopropyl-5-ethylhexane4. 3-methyl-4-isopropylheptane5. 4-isopropyl-3-methylheptane
SolutionSolution
What is the systematic name of the alkane shown What is the systematic name of the alkane shown below?below?
1. 3,7-dimethyl-4-propyloctane2. 2-methyl-5-(1-methylpropyl)octane3. 2-ethyl-6-methyl-3-propylheptane4. 3-methyl-4-(3-methylbutyl)heptane5. 2,6-dimethyl-5-propyloctane
Learning CheckLearning Check
What is the systematic name of the alkane shown What is the systematic name of the alkane shown below?below?
1. 3,7-dimethyl-4-propyloctane2. 2-methyl-5-(1-methylpropyl)octane3. 2-ethyl-6-methyl-3-propylheptane4. 3-methyl-4-(3-methylbutyl)heptane5. 2,6-dimethyl-5-propyloctane
SolutionSolution
Which of the following Which of the following is notis not an isomer of an isomer of hexane?hexane?
1. 2,2-dimethylbutane2. 2,2-dimethylpentane3. 3-methylpentane4. 2-methylpentane5. 2,3-dimethylbutane
Learning CheckLearning Check
Which of the following Which of the following is notis not an isomer of an isomer of hexane?hexane?
1. 2,2-dimethylbutane2. 2,2-dimethylpentane3. 3-methylpentane4. 2-methylpentane5. 2,3-dimethylbutane
SolutionSolution
How many constitutional isomers of How many constitutional isomers of CC44HH1010O are alcohols?O are alcohols?
1. 12. 23. 34. 45. 5
Learning CheckLearning Check
How many constitutional isomers of How many constitutional isomers of CC44HH1010O are alcohols?O are alcohols?
1. 12. 23. 34. 45. 5
SolutionSolution
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3.5 Properties of Alkanes3.5 Properties of AlkanesCalled paraffins (low affinity compounds)
because they do not react as most chemicalsThey will burn in a flame, producing carbon
dioxide, water, and heat
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Crude OilCrude OilThe hydrocarbons
in crude oil areSeparated by
boiling points.Heated to
higher temperatures to produce gases that can be removed and cooled.
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alkane + O2 CO2 + H2O + energy
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Combustion of AlkanesCombustion of AlkanesAlkanes Undergo combustion
by reacting with oxygen to produce carbon dioxide, water, and energy.
Are typically not very reactive due to strong C-C single bonds.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
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Learning CheckLearning Check
Propane is used to provide heat forcooking or warming a room. Write abalanced equation for the completecombustion of propane.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
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Learning CheckLearning CheckPropane is used to provide heat forcooking or warming a room. Write abalanced equation for the completecombustion of propane.
Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings
SolutionSolution
C3H8 + 5O2 3CO2 + 4H2O
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Write a balanced equation for the reaction for the complete combustion of:
heptane
ethane
Learning CheckLearning Check
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Write a balanced equation for the reaction for the complete combustion of:
heptane C7H16.
ethane
SolutionSolution
CC77HH1616 + 11O + 11O22 7CO 7CO22 + 8H + 8H22OO
2C2C22HH66 + 7O + 7O22 4CO4CO22 + 6H + 6H22O O
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Properties of AlkanesProperties of AlkanesThey react with Halogens like Cl2 in the
presence of light to replace H’s with Cl’s (not controlled)
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Learning CheckLearning CheckGive the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light.
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SolutionSolutionGive the structures and names of the possible monosubstituted products for the reaction of propane with bromine in the presence of light.
CH3—CH2—CH2— Br
1-bromopropane; propyl bromide
Br
|
CH3—CH—CH3
2-bromopropane; isopropyl bromide
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Physical PropertiesPhysical PropertiesBoiling points and melting points increase as size of alkane
increasesDispersion forces increase as molecule size increases,
resulting in higher melting and boiling points
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3.6 Conformations of Ethane3.6 Conformations of EthaneStereochemistry concerned with the 3-D
aspects of moleculesσ bonds are cylindrically symmetricalRotation is possible around C-C bonds in
open-chain molecules
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ConformersConformersConformation- Different arrangement
of atoms resulting from bond rotationConformations can be represented in 2
ways:
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Torsional StrainTorsional Strain
We do not observe perfectly free rotationThere is a barrier to rotation, and some
conformers are more stable than othersStaggered- most stable: all 6 C-H bonds
are as far away as possibleEclipsed- least stable: all 6 C-H bonds are
as close as possible to each other
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3.7 Conformations of Other Alkanes3.7 Conformations of Other AlkanesThe eclipsed conformer of propane has 3
interactions: two ethane-type H-H interactions, and one H-CH3 interaction
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• Conformational situation is more complex for larger alkanes
• Not all staggered conformations has same energy, and not all
eclipsed conformations have same energy
Conformations of Other Alkanes
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Conformations of ButaneConformations of ButaneAnti conformation- methyl groups are 180˚ apartGauche conformation- methyl groups are 60˚ apart
Which is the most energetically stable?
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Steric StrainSteric StrainSteric strain- repulsive interaction occurring between atoms
that are forced closer together than their atomic radii allow
6
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3.8 1111
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What is the correct order of stability, starting with What is the correct order of stability, starting with the most strained, for the conformations shown the most strained, for the conformations shown below?below?
1. A B, C2. A, C, B3. B, A, C4. C, A, B5. C, B, A
CH3
H H
CH2CH3
HH3C
CH3
H HCH3
CH2CH3HCH3
H HH
CH3CH3CH2
A B C
Learning CheckLearning Check
What is the correct order of stability, starting with What is the correct order of stability, starting with the most strained, for the conformations shown the most strained, for the conformations shown below?below?
1. A B, C2. A, C, B3. B, A, C4. C, A, B5. C, B, A
CH3
H H
CH2CH3
HH3C
CH3
H HCH3
CH2CH3HCH3
H HH
CH3CH3CH2
A B C
SolutionSolution
CC66HH1414 has 5 isomers. How many of these has 5 isomers. How many of these isomers have at least one tertiary carbon?isomers have at least one tertiary carbon?
1. 02. 13. 24. 35. 4
Learning CheckLearning Check
CC66HH1414 has 5 isomers. How many of these has 5 isomers. How many of these isomers have at least one tertiary carbon?isomers have at least one tertiary carbon?
1. 02. 13. 24. 35. 4
SolutionSolution
Which statement about bond rotations around C-C Which statement about bond rotations around C-C bonds in ethane and ethylene is correct?bonds in ethane and ethylene is correct?
1. Rotation around σ bond is easy because the orbital overlap does not change with rotation.
2. Rotation around π bond is easy because there are p orbitals available in all directions of space.
3. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other.
4. Rotation around π bond is hard because the electrons are delocalized.
Learning CheckLearning Check
Which statement about bond rotations around C-C Which statement about bond rotations around C-C bonds in ethane and ethylene is correct?bonds in ethane and ethylene is correct?
1. Rotation around σ bond is easy because the orbital overlap does not change with rotation.
2. Rotation around π bond is easy because there are p orbitals available in all directions of space.
3. Rotation around σ bond is very hard because the electron pairs from the adjacent bonds must pass by each other.
4. Rotation around π bond is hard because the electrons are delocalized.
SolutionSolution
Which of the above represents the Newman projection of Which of the above represents the Newman projection of 2-bromo-3-chlorobutane?2-bromo-3-chlorobutane?
1. a2. b3. c4. d5. e
CH3
H
H
H3C
Br
Cl
H
CH3
H
Br
CH3
Cl
Cl
CH3
H
H
Br
CH3
CH3
Cl
H3C
H
Br
H
H
CH3
Br
H3C
CH3
Cl
a) b) c) d) e)
Learning CheckLearning Check
Which of the above represents the Newman projection of Which of the above represents the Newman projection of 2-bromo-3-chlorobutane?2-bromo-3-chlorobutane?
1. a2. b3. c4. d5. e
CH3
H
H
H3C
Br
Cl
H
CH3
H
Br
CH3
Cl
Cl
CH3
H
H
Br
CH3
CH3
Cl
H3C
H
Br
H
H
CH3
Br
H3C
CH3
Cl
a) b) c) d) e)
SolutionSolution
The lowest energy conformation of decane resembles a The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation the minimum strain that such a once-bent conformation (bent rod) must have?(bent rod) must have?
1. 3.8 kJ/mol2. 4.0 kJ/mol3. 6.0 kJ/mol4. 0.0 kJ/mol5. 11.0 kJ/mol
Learning CheckLearning Check
The lowest energy conformation of decane resembles a The lowest energy conformation of decane resembles a straight rod. It is a fully stretched “zig-zag” conformation straight rod. It is a fully stretched “zig-zag” conformation with all bonds staggered and all large substituents anti to with all bonds staggered and all large substituents anti to each other. Imagine we bend this chain at carbon 5. What is each other. Imagine we bend this chain at carbon 5. What is the minimum strain that such a once-bent conformation the minimum strain that such a once-bent conformation (bent rod) must have?(bent rod) must have?
1. 3.8 kJ/mol2. 4.0 kJ/mol3. 6.0 kJ/mol4. 0.0 kJ/mol5. 11.0 kJ/mol
SolutionSolution
What is the energy difference between the two lowest-What is the energy difference between the two lowest-energy conformations of 2,3-dimethylbutane?energy conformations of 2,3-dimethylbutane?
1. the energy of one gauche interaction2. the energy of two gauche interactions3. the energy of two H ↔ CH3 eclipsing interactions
4. the energy of three H ↔ CH3 eclipsing interactions
5. the energy of two CH3 ↔ CH3 eclipsing interactions
Learning CheckLearning Check
What is the energy difference between the two lowest-What is the energy difference between the two lowest-energy conformations of 2,3-dimethylbutane?energy conformations of 2,3-dimethylbutane?
1. the energy of one gauche interaction2. the energy of two gauche interactions3. the energy of two H ↔ CH3 eclipsing interactions
4. the energy of three H ↔ CH3 eclipsing interactions
5. the energy of two CH3 ↔ CH3 eclipsing interactions
SolutionSolution
The highest energy conformation of ethane has 12 kJ/mol of The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have?highest energy conformation of 2,3-dimethylbutane have?
1. 16 kJ/mol2. 20 kJ/mol3. 26 kJ/mol4. 31 kJ/mol5. 37 kJ/mol
Learning CheckLearning Check
The highest energy conformation of ethane has 12 kJ/mol of The highest energy conformation of ethane has 12 kJ/mol of strain. The two highest conformations of butane have 16 and strain. The two highest conformations of butane have 16 and 19 kJ/mol of strain, respectively. How much strain does the 19 kJ/mol of strain, respectively. How much strain does the highest energy conformation of 2,3-dimethylbutane have?highest energy conformation of 2,3-dimethylbutane have?
1. 16 kJ/mol2. 20 kJ/mol3. 26 kJ/mol4. 31 kJ/mol5. 37 kJ/mol
SolutionSolution