Chapter 3 Flashcards

1
Q

Boiling Point of Alkanes

A

As the number of carbons in an alkane increases, the boiling point will increase due to the larger surface area and the increased Van der Waal attractions

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2
Q

Melting points of Alkanes

A
  • Melting points increase as the carbon chain increases
  • Alkanes with an even number of carbons have higher melting points than those with an odd number of carbons
  • Branched alkanes have higher melting points than unbranched alkanes
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3
Q

Methane Representation

A
  • Tetrahedral
  • sp3 hybrid carbon with angles of 109.5 degrees
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4
Q

Ethane Representations

A
  • Two sp3 hybrid carbons
  • Rotation about the C-C sigma bond
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5
Q

Conformations definition

A

Conformations are different arrangements of atoms caused by rotation about a single bond

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6
Q

Conformations of Ethane

A

Pure confomers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations

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7
Q

ethane eclipsed conformation

A
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8
Q

ethane staggered conformation

A
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9
Q

Ethane Conformations Energy Diagram

A

The torsional energy of ethane is lowest in the staggered conformation

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10
Q

Propane Conformations

A
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11
Q

propane eclipsed conformation

A
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12
Q

propane staggered conformation

A
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13
Q

Propane conformation energy diagram

A

The staggered conformations of propane are lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, there is a greater torsional strain for propane than for ethane.

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14
Q

butane totally eclipsed conformation

A
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15
Q

butane gauche (staggered) conformation

A
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16
Q

butane eclipsed conformation

A
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17
Q

butane anti (staggered) conformation

A
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18
Q

Steric Strain in Butane

A
  • The totally eclipsed conformation is higher in energy because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion
  • This kind of interference between two bulky groups is called steric strain
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19
Q

Butane conformation energy diagram

A
20
Q

Geometric Isomers

A
21
Q

Torsional Strain

A
  • Torsional strain reflects barrier to rotating about the C-C single bond.
  • For butane, high torsional energy for the totally eclipsed conformer is due to steric strain (two groups trying to occupy the same space).
22
Q

Angle Strain in Cycloalkanes

A
  • When a cycloalkane has an angle other than 109.5º, there will not be optimum overlap and the compound will have angle strain.
  • This is because all the C atoms are sp3 hybridized
23
Q

Torsional Strain in Cycloalkanes

A
  • Torsional strain arises when all the bonds are eclipsed
24
Q

Ring Strain in Cycloalkanes

A
  • Ring strain reflects contributions of angle strain and torsional strain.
25
Q

Cyclopropane

A
  • The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms
  • This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds” (sometimes called banana bonds)
  • Leads to interesting reactivity.
26
Q

Torsional strain in cyclopropane

A
  • All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.
27
Q

Planar Cyclobutane

A

In a planar conformation:

  • Angle strain from the compressing of the bond angles to 90°
  • Torsional strain from eclipsing of the bonds
28
Q

Non-planar Cyclobutane

A
  • Cyclic compound with four carbons or more adopt non-planar conformations to relieve torsional strain.
  • Cyclobutane adopts the folded conformation (“envelope”) to decrease the torsional strain caused by eclipsing hydrogens.
29
Q

Cyclopentane

A
  • The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent methylene (CH2) groups.
30
Q

Cyclohexane Chair Conformation

A
31
Q

Cyclohexane Boat Conformation

A
32
Q

Conformational Energy Diagram of Cyclohexane

A
33
Q

Axial and Equitorial Positions

A
34
Q

Chair-Chair Interconversion

A
  • Chair conversion results in switching all the axial substituents into the equatorial positions, and vice versa.
35
Q

Equitorial Methyl Group

A
36
Q

Axial Methyl in Methylcyclohexane

A
37
Q

Tert-butylcyclohexane

A
  • Very large substituents (like t-butyl) will ONLY occupy equatorial positions
  • Chair will be locked in this conformation
38
Q

Cis-1,3-dimethylcyclohexane

A
  • Cis-1,3-dimethylcyclohexane can have both methyl groups in axial positions or both in equatorial positions.
  • The conformation with both methyl groups equatorial is more stable.
39
Q

Tert-butylcyclohexane

A
  • Very large substituents (like t-butyl) will ONLY occupy equatorial positions
  • Chair will be locked in this conformation
40
Q

Cis-1,4-ditertbutylcyclohexane

A

The most stable conformation of cis-1,4-di-tertbutylcyclohexane is the twist boat. Both chair conformations require one of the bulky t-butyl groups to occupy an axial position.

41
Q

cis - 1,2-dimethylhexane positions

A
42
Q

trans - 1,2-dimethylhexane positions

A
43
Q

cis - 1,3-dimethylhexane positions

A
44
Q

trans - 1,3-dimethylhexane positions

A
45
Q

cis - 1,4-dimethylhexane positions

A
46
Q

trans - 1,4-dimethylhexane positions

A