Chapter 3 Flashcards Preview

Organic Chemistry 1 > Chapter 3 > Flashcards

Flashcards in Chapter 3 Deck (46)
Loading flashcards...
1

Boiling Point of Alkanes

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

2

Melting points of Alkanes

- 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

3

Methane Representation

- Tetrahedral

- sp3 hybrid carbon with angles of 109.5 degrees

4

Ethane Representations

- Two sp3 hybrid carbons

- Rotation about the C-C sigma bond

5

Conformations definition

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

6

Conformations of Ethane

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

7

ethane eclipsed conformation

8

ethane staggered conformation

9

Ethane Conformations Energy Diagram

The torsional energy of ethane is lowest in the staggered conformation

10

Propane Conformations

11

propane eclipsed conformation

12

propane staggered conformation

13

Propane conformation energy diagram

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.

14

butane totally eclipsed conformation

15

butane gauche (staggered) conformation

16

butane eclipsed conformation

17

butane anti (staggered) conformation

18

Steric Strain in Butane

- 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

 

19

Butane conformation energy diagram

20

Geometric Isomers

21

Torsional Strain

- 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

Angle Strain in Cycloalkanes

- 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 sphybridized

23

Torsional Strain in Cycloalkanes

- Torsional strain arises when all the bonds are eclipsed

24

Ring Strain in Cycloalkanes

- Ring strain reflects contributions of angle strain and torsional strain.

25

Cyclopropane

- 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

Torsional strain in cyclopropane

- All the C—C bonds are eclipsed, generating torsional strain that contributes to the total ring strain.

27

Planar Cyclobutane

In a planar conformation:

 - Angle strain from the compressing of the bond angles to 90°

- Torsional strain from eclipsing of the bonds

28

Non-planar Cyclobutane

- 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

Cyclopentane

- 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

Cyclohexane Chair Conformation