Chapter 5: 5.3 Alkanes, Cycloalkanes, and Alkenes

Question 1

Define:
Alkanes

Answer 1

Straight or branched-chained compounds containing C and H atoms and only single carbon-carbon bonds

Question 2

State the general formula for acyclic alkanes

Answer 2

C(n)H(2n+2)

Question 3

Define:
Homologous series

Answer 3

Compounds that are structurally related to one another

Question 4

Define:
Constitutional isomers

Answer 4

Compounds with the same molecular formula but different bonding sequences

Question 5

What are constitutional isomers also known as?

Answer 5

Structural isomers

Question 6

True or False:
Physical properties of alkanes change with molecular size

Answer 6

True

Question 7

True or False:
Constitutional isomers have the same chemical properties

Answer 7

False, they have different chemical and physical properties

Question 8

What are the 3 common ways of representing 3D Structures?

Answer 8

Dot-line-wedge formulas
Sawhorse projections
Newman projections

Question 9

Describe dot-line-wedge formulas

Answer 9

Lines represent bonds in the plane of the paper
Dotted lines represent bonds behind the plane of the paper
Wedged lines represent bonds in front of the plane of the paper

Question 10

Describe sawhorse projections

Answer 10

Drawn in the shape of a sawhorse using only plain lines

Question 11

What molecules are sawhorse projections used for?

Answer 11

Only for simple molecules

Question 12

Describe Newman projections

Answer 12

The molecule is viewed along the C-C axis bonds are drawn in front of and along the edge of a circle

Question 13

Define:
Conformations

Answer 13

Transient (non-permanent) spatial arrangements of atoms in a molecule produced by rotations about single bonds

Question 14

Can conformations occur around double bonds?

Answer 14

No, as they cannot spin around double bonds

Question 15

What are the two key conformations of alkanes?

Answer 15

Staggered
Eclipsed

Question 16

Describe staggered conformation

Answer 16

The C-H- bonds in front bisect the H-C-H bond angles in the back (C-H bonds are not aligned)

Question 17

Describe eclipsed conformation

Answer 17

All C-H bonds are aligned

Question 18

Eclipsed conformation is ______ in energy because:

Answer 18

Higher
Steric repulsion between the front and rear H atoms

Question 19

Are conformations the same as isomers?

Answer 19

No, conformations can switch seamlessly back and forth
Isomers are unique

Question 20

Since the energy difference between staggered and eclipsed conformations are relatively _____, they interconvert at a ____ rate

Answer 20

Small
Fast

Question 21

What is the angle between methyl groups in a Newman projection known as?

Answer 21

Dihedral angle

Question 22

When there are multiple staggered conformations, what is the one lowest in energy called?

Answer 22

Anti conformer

Question 23

When there are multiple staggered conformations, what are the ones higher in energy than the anti conformer called?

Answer 23

Gauche conformers

Question 24

Define:
Cycloalkanes

Answer 24

Hydrocarbons containing a ring of carbon atoms connected by single C-C bonds

Question 25

What is the general formula for cycloalkanes?

Answer 25

C(n)H(2n)

Question 26

Of all cycloalkanes, which one is the most common?

Answer 26

Cyclohexane

Question 27

What are the two distinct conformations that cyclohexane has?

Answer 27

Boat Chair

Question 28

Why is the boat conformation of cyclohexane present in very small amounts?

Answer 28

The flagpole interactions due to eclipsed conformations make it not favorable

Question 29

Describe conformations for C-H bonds in a chair conformation

Answer 29

All C-H bonds on adjacent carbons are staggered

Question 30

Define: Axial substituents

Answer 30

The six substituents directed straight up or down in chair conformation of cyclohexane

Question 31

Define: Equatorial susbtituents

Answer 31

The other six substituents directed laterally or outward in chair conformation of cyclohexane

Question 32

Large groups prefer to be in an ___________ position. Why?

Answer 32

Equatorial Equatorial substituents are less crowded since they are more distant from other substituents in the ring

Question 33

Define: Constitutional isomers

Answer 33

Isomers that share the same molecular formula but cannot be formed by rotating one to form the other

Question 34

Define: Stereoisomers

Answer 34

Cis-trans isomers Occurs when a cycloalkane ring has two or more non-hydrogen substituents on different ring carbons, they can occupy two different positions relative to the approximate ring plane

Question 35

What is the difference between stereoisomers and constitutional isomers?

Answer 35

Both have the same molecular formulas Constitutional isomers have a different bond order

Question 36

Describe cis isomers

Answer 36

Two substituents on the same (cis) side of the ring plane

Question 37

Describe trans isomers

Answer 37

Two substituents on opposite (trans) sides of the ring plane

Question 38

Stereoisomers have same _________ ________ and the same _______ ________ (____________), but have different, non-interconverting __ __________

Answer 38

Molecular formula Bonding sequence (connectivity) 3D structures

Question 39

Do cis-trans isomers interconvert at room temperature?

Answer 39

No

Question 40

Define: Alkenes

Answer 40

Hydrocarbons with one or more double bonds

Question 41

State the general formula of homologous alkenes with one C=C double bond

Answer 41

C(n)H(2n)

Question 42

Define: Alkynes

Answer 42

Hydrocarbons with one or more triple bonds

Question 43

State the general formula of homologous alkynes with CC triple bond

Answer 43

C(c)H(2n-2)

Question 44

What does it mean for a hydrocarbon to be saturated?

Answer 44

It contains the maximum possible number hydrogen atoms (2n+2)

Question 45

Select the unsaturated hydrocarbon(s) from below: Alkanes Alkenes Alkynes

Answer 45

Alkenes and alkynes

Question 46

The removal of _ H atoms represent ___ unit of unsaturation

Answer 46

2 One

Question 47

What is the structure of an alkene? Explain

Answer 47

Planar structure The C=C bond is bonded to three atoms and is sp2 (planar), the remaining p orbital overlap to form a pi bond which cannot rotate, leaving it in planar

Question 48

What is the structure of an alkyne? Explain

Answer 48

Linear structure The CC triple bond is bonded to two atoms and is sp (linear), the remaining 2 p orbitals give rise to 2 pi orbitals which cannot rotate, leaving it in linear

Question 49

What is a potential consequence of the inability of the carbon-carbon double bond in alkenes to rotate?

Answer 49

Geometric isomerism

Question 50

True or False: cis-trans arrangements can occur in rings

Answer 50

Not exactly In rings with seven members or less, a double bond within the ring can have only the cis geometry because the trans arrangement would be too strained

Question 51

State the formula for maximum possible number of cis-trans isomers

Answer 51

2^n n is the number of double bonds