1 CH 3: Organic Compounds: Alkanes and Their Stereochemistry Renee Y. Becker CHM 2210 Valencia Community College
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
1
CH 3: Organic Compounds: Alkanes and Their Stereochemistry
Renee Y. Becker
CHM 2210
Valencia Community College
2
Why this Chapter
• Alkanes 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
3
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
4
Functional Groups with Multiple Carbon–Carbon Bonds
• Alkenes have a C-C double bond
• Alkynes 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 & bonds?
5
Functional Groups with Carbon Singly Bonded to an Electronegative Atom
6
Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)
7
8
9
10
11
Alkanes
• Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)
• Connecting carbons can lead to large or small molecules
• The formula for an alkane with no rings in it must be CnH2n+2 where the number of C’s is n
• Alkanes are saturated with hydrogen (no more can be added
• They are also called aliphatic compounds
12
13
Naming Alkanes
Memorize 1-10 for next class
14
Alkane Isomers
• 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
15
Constitutional Isomers
• Isomers that differ in how their atoms are arranged in chains are called constitutional isomers
• Compounds other than alkanes can be constitutional isomers of one another
• They must have the same molecular formula to be isomers
16
Condensed Structures of Alkanes
• We can represent an alkane in a brief form or in many types of extended form
• A condensed structure does not show bonds but lists atoms, such as – CH3CH2CH2CH3 (butane)– CH3(CH2)2CH3 (butane)
17
Alkyl Groups
• Alkyl 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
18
Types 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)
19
Types of Hydrogens
20
Substituents
21
Common Names
• Isobutane, “isomer of butane”
• Isopentane, isohexane, etc., methyl branch on next-to-last carbon in chain.
• Neopentane, most highly branched
• Five possible isomers of hexane,18 isomers of octane and 75 for decane!
22
Pentanes
CH
H
C
H
H
C
H
H
C
H
C
HH
H
H
H
n-pentane, C5H12
CHH
C CHH
C H
H
H
HH
CH
HH
isopentane, C5H12
C
CH3
CH3H3C
CH3
neopentane, C5H12
23
IUPAC Names
• Find the longest continuous carbon chain.
• Number the carbons, starting closest to the first branch.
• Name the groups attached to the chain, using the carbon number as the locator.
• Alphabetize substituents.
• Use di-, tri-, etc., for multiples of same substituent.
24
Longest Chain
• The number of carbons in the longest chain determines the base name: ethane, hexane.
• If there are two possible chains with the same number of carbons, use the chain with the most substituents.
C
CH3
CH2
CH3
CH CH2 CH2 CH3
CH CH2 CH3H3C
H3C
25
Number the Carbons
• Start at the end closest to the first attached group.
• If two substituents are equidistant, look for the next closest group.
1
2
3 4 5
6 7CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
26
Name Alkyl Groups
• CH3-, methyl
• CH3CH2-, ethyl
CH3 CH CH2 CH3
sec-butyl
CH3 CH
CH3
CH2
isobutyl
CH3 CH CH3
isopropyl
CH3C
CH3
CH3
tert-butyl
• CH3CH2CH2-, n-propyl
• CH3CH2CH2CH2-, n-butyl
27
Propyl Groups
C
H
H
H
C
H
H
C
H
H
H
n-propyl
C
H
H
H
C
H
C
H
H
H
isopropyl
H
A primary carbon A secondary carbon
28
Butyl Groups
C
H
H
H
C
H
C
H
H
C
H
H
H
C
H
H
H
C
H
C
H
HH
C
H
H
n-butyl sec-butyl
H
H
A primary carbon A secondary carbon
29
Isobutyl Groups
CH
H
H
C
CH H
C
HH
H H
CH
H
H
C
CH H
C H
HH
HH
H
A primary carbon A tertiary carbon
isobutyl tert-butyl
30
Alphabetize
• Alphabetize substituents by name.
• Ignore di-, tri-, etc. for alphabetizing.
CHH3C
CH3
CH
CH2CH3
CH2 CH2 CH
CH3
CH3
3-ethyl-2,6-dimethylheptane
31
Example 1
Write structures for the following:
a) 3-ethyl-3-methylpentane
b) 4-t-butyl-2-methylheptane
c) 5-isopropyl-3,3,4-trimethyloctane
d) 3-ethyl-2,4,5-trimethylheptane
32
Example 2
Provide IUPAC & common names (1-3)
1
2
3
4
5
6
1
2
3
4
5
6
33
Example 3Name the following
a) 3,3-dimethyl-4-isobutylheptane
b) 3,3-dimethyl-4-tert-butylheptane
c) 4-tert-butyl-3,3-dimethylheptane
d) 4-isobutyl-3,3-dimethylheptane
34
Complex Substituents
• If the branch has a branch, number the carbons from the point of attachment.
• Name the branch off the branch using a locator number.
• Parentheses are used around the complex branch name.
• For alphabetizing use the first letter of the complex sub. Even if it is a numerical (di, tri, etc)
Complex Examples
35
a
b
c
d
a
b
c
d
36
Example 4
Draw the structures and give their more common names
a) (1-methylethyl) group
b) (2-methylpropyl) group
c) (1-methylpropyl) group
d) (1,1-dimethylethyl) group
37
Example 5
Draw the structures
a)4-(1-methylethyl)heptane
b) 5-(1,2,2-trimethylpropyl)nonane
38
Assignment
• For next class draw the 9 isomers of heptane and name them
39
Properties of Alkanes
• Called paraffins (low affinity compounds) because they do not react as most chemicals
• They will burn in a flame, producing carbon dioxide, water, and heat
• They react with Cl2 in the presence of light to replace H’s with Cl’s (not controlled)
40
Physical Properties: Alkanes
• Solubility: hydrophobic
• Density: less than 1 g/mL
• Boiling points increase with increasing carbons (little less for branched chains).
• Melting points increase with increasing carbons (less for odd-number of carbons).
41
Boiling Points of Alkanes
Branched alkanes have less surface area contact,so weaker intermolecular forces.
42
Melting Points of Alkanes
Branched alkanes pack more efficiently into a crystalline structure, so have higher m.p.
43
Branched Alkanes
• Lower b.p. with increased branching
• Higher m.p. with increased branching
H
CH3CH
CH3
CH2 CH2 CH3
bp 60°Cmp -154°C
CH3CH
CH3
CHCH3
CH3 bp 58°Cmp -135°C
bp 50°Cmp -98°C
CH3 C
C 3
CH3
CH2 CH3
44
Example 6
List each set of compounds in order of increasing boiling point and melting point
a
b
a
b
45
Example 7
Which of the following has the highest boiling point?
a) 3-methylpentane
b) 2,2-dimethylbutane
c) Hexane
d) Methane
46
Conformers
• Conformation- Different arrangement of atoms resulting from bond rotation
• Conformations can be represented in 2 ways:
47
Torsional Strain
• We do not observe perfectly free rotation
• There is a barrier to rotation, and some conformers are more stable than others
• Staggered- most stable: all 6 C-H bonds are as far away as possible
• Eclipsed- least stable: all 6 C-H bonds are as close as possible to each other
48
Conformations of Ethane
• Stereochemistry concerned with the 3-D aspects of molecules
• Rotation is possible around C-C bonds in open-chain molecules (not cyclic)
49
Ethane Conformers • Eclipsed conformer has highest energy• Dihedral angle = 0 degrees
50
Conformational Analysis
• Torsional strain: resistance to rotation.• For ethane, only 3.0 kcal/mol
51
Propane Conformers
Note slight increase in torsional strain due to the more bulky methyl group.
52
Butane Conformers C2-C3
• Highest energy has methyl groups eclipsed.• Steric hindrance• Dihedral angle = 0 degrees
totally eclipsed
53
Butane Conformers (2)
• Lowest energy has methyl groups anti.• Dihedral angle = 180 degrees
anti
54
Butane Conformers (3)
• Methyl groups eclipsed with hydrogens• Higher energy than staggered conformer• Dihedral angle = 120 degrees
eclipsed
55
Butane Conformers (4)
• Gauche, staggered conformer• Methyls closer than in anti conformer• Dihedral angle = 60 degrees
gauche
56
Conformational Analysis
57
Higher Alkanes
• Anti conformation is lowest in energy.• “Straight chain” actually is zigzag.
CH3CH2CH2CH2CH3
CH C
CCC
H H H H
H H
H H
HH H
58
Related Documents
