sevada chamras, ph.d. glendale community …4 indicate the location of the substituent(s) [side...
TRANSCRIPT
1
Sevada Chamras, Ph.D.
Glendale Community College
Chemistry 105
Exam. 1 Lecture Notes
Chapters 1, 2, & 3
________________________________________________________________________________________
CHAPTER 3
*** 3-D Molecular Model Set Needed***
Saturated Hydrocarbons (AKA: Alkanes) (AKA:Paraffins)
1. General Formula:
2. “Functional Group” or “Side Chain”:
2. General IUPAC Nomenclature for Alkanes:
C's Molecular Formula ! Name Functional GroupFormula ! Name
1 CH4 ! –CH3 !
2 ! !
3 ! !
4 ! !
5 ! !
6 ! !
7 ! !
8 ! !
9 ! !
10 ! !
11 ! !
12 ! !
2
***Note: Only 4 bonds are extended from carbons, and only 1 bond is extended from
hydrogens.
3. Isomerism in Hydrocarbons:
n-alkanes:
Constitutional isomers of n-alkanes:
EXAMPLE:
Methane: (1)
Ethane: (1)
Propane: (1)
Butane: (2)
Pentane: (3)
Hexane: (5)
How many isomers? Is there a predicting formula?
3
4. IUPAC Nomenclature Steps and Rules for Alkanes:
• Locate the longest continuous chain! This will be used as the name of the base compound
Example:
***What if there is more than 1 longest continuous chain?
Example:
• Number the carbons in the main chain.
***Note: There are 2 ways (from 2 ends) of numbering the chain. Choose the end as the #1
carbon that is nearest to the substituent.
4
Indicate the location of the substituent(s) [side chain(s)].
When there is more than one side chain, the correct way for naming should have the
side-chains mentioned by alphabetical order.
When two or more of the side chains are the same, use numbering prefixes like di, tri,
tetra, etc. …, in order to avoid repetition of the names for the side chains.
Numbers are separated from each other with commas.
Numbers are separated from letters with dashes.
Types of Carbons Based on Substitution:
1. Methyl:
2. Primary:
3. Secondary:
4. Tertiary:
5
Some Common Side Chains:
CH3
CH
H3C
CH3CH3C
CH3H3C
CH
H2C
CH3
H3C
CH2
H2C
H3C
H2C
CH2
H2C
H3C
H2C
CH
CH3
6
Examples for Naming More Complex Side Chains:
__________________________________________________________________________
Physical Properties of Alkanes:
a) Solubility (Polarity):
b) Average Density:
c) Melting Point:
What Happens at Melting Point?
1. The Effect of Molar Mass:
CH CH
CH3CH2H3C
VCH3
H3C
C
H2C
CH
CH3
CH3CH3
7
2. The Odd-Even Carbon Effect:
3. The Effect of Branching:
___________________________________________________________________________
Structure & Conformation of Alkanes:
1. Hybridization and Structure of Carbons in Alkanes:
Hybridization ! Bonds ! Geometry ! Ideal Bond Angle
2. What is Conformation?
3. What is Free Rotation?
4. What is Conformational Isomer? (Conformer)
8
5. An Initial Look at Ethane and Its Two Possible Conformations:
***Drawing 3-Dimensional (Wedge-Dash) Structures: (Use the model set now)
Drawing Tools
a) Draw the carbon skeleton in the plane of the paper, showing the carbons:
Ethane Propane
CH3H3C
C2H6Molecular Formula:
Condensed Formula: H3CCH3
Bond-Line Formula:
Expanded Structural Formula: C C
H
H
H
H
H
H
3-Dimensional Structural Formula:
(AKA: Wedge-Dash)
Wedge DashLine
9
b) Add in the hydrogens using the wedges, the dashes, and the straight lines:
Ethane Propane
6. Two Methods for Structure Drawing to Study Conformers More Effectively:
a) Newman Projection: (Viewing the molecule along the C-C bond axis)
• Draw two possible Wedge-Dash structure for Ethane:
Structure A Structure B
• Convert the Wedge-Dash to Newman Projection by looking at it through the C-C
bond of ethane: (Construct the model and view it along the C-C axis)
Structure A Structure B
10
Names for Structures (conformers) A and B, based on the relative orientations of the hydrogen
atoms on the carbons:
Structure A:
Structure B:
Energetic Relation between Eclipsed and Staggered Conformers:
Reason for the Energetic Trend:
• Dihedral Angle (! ): Definition: ___________________________________
b) Sawhorse Projection:
• Start with the Newman Projection for each conformer drawn above:
• In your mind (or using the model set now) rotate the molecule thus tilting it
enough to be able to see the C-C bond:
• Draw what you see:
Eclipsed Staggered
11
Conformational Analysis of Ethane: Study of the change in the potential energy of ethane as
a function of rotation around C-C bond (change in the dihedral angle size):
Dihedral Angle (o)
Conformational Analysis of Butane:
Dihedral Angle (o)
New Words:
Gauche Conformer:
Anti Conformer:
Tortional Strain (Tortional Energy):
Steric Strain (Steric Energy):
E
0o 60o 120o 180o
E
0o 60o 120o 180o 240o 300o 360o
12
Cyclic Saturated Hydrocarbons (AKA: Cycloalkanes)
• General Formula:
• Naming Unsubstituted Cyclic Alkanes:
___________________ ___________________ ___________________
___________________ ___________________ ___________________
• Naming Substituted Cyclic Alkanes:
1. With one substituent: Similar to open-chain alkanes:
Example:
___________________ ___________________
2. With more than one substituent:
13
• Number the ring carbons. The correct direction for numbering is the one that results in
the smallest numbers for the substituents:
Example:
________________________
________________________
• When the open chain contains more carbons than the ring, the open chain becomes the
main compound, and the ting becomes a substituent (side chain).
Example:
________________________
__________________________________________________________________________
14
Conformations in Cycloalkanes:
1. Cyclopropane: An equilateral triangle (seemingly). Angles: 60o vs. 72
o.
Bent Bonds:
Strengtth of the C-C bonds:
Angle Strain: sp3 angle = 109.5
o, C-C-C angle in cyclopropane =
2. Cyclobutane: Flat vs. puckered ring:
Angles = 90o
109.5o>Angles > 90
o
3. Cyclopentane: Angle = 108o
Three conformations:
Planar Envelope Half-Chair
More torsional Less Torsional Less Torsional
Strain Strain Strain
60o
15
4. Cyclohexane: Virtually free of torsional and angle strain.
Conformations: Chair, boat, twist-boat.
Drawing the Chair Conformer
a) Drawing the carbon skeleton:
1. Draw 2 parallel lines as shown:
2. Draw another set of two parallel lines stemming from the end of one and the beginning
of the other line:
3. Connect the two bent shapes to complete the carbon skeleton of the ring:
b) Drawing in the substituents (hydrogens in case of an unsubstituted cyclohexane):
1. The axial substituents are drawn as vertical lines pointing above and below the plane of
the ring in an alternating fashion.
Hint: Start with the higher tip of the ring and have the substituent pointing above the plane
of the ring.
H
H
H
H
H
H
16
2. The equatorial substituents are drawn as tilted straight lines pointing above and below the
plane of the ring, and directed away from the body of the ring. If the axial substituent points
above the plane, then the axial should point below, and vice versa.
*Hint: To get a better, more precise idea on the exact direction of the equatorial substituents, 4
of the 6 equatorial substituents are parallel to 4 C-C bonds of the ring. The picture below
illustrated the corresponding parallel pairs:
Conformation Conversions in Cyclohexane:
Chair 1 "! Half-Chair 1 "! Twist Boat "! Half-Chair 2 "! Chair 2
Energetic comparison of the conformers:
.Chairs:
.Half-Chairs:
.Twist Boat: E
Boat
H
H
H
H
H
H
HH
H
H
H
H
H
H
H
H
H
H
HH
H
H
H
H
1
1
2
2
3
3
4
4
17
Flipping Chairs!
Substituted Cyclohexanes:
1,3-Diaxial Interaction:
Equilibrium between the chair forms of mono-substituted cyclohexanes:
1
4
7
12
9
6
211
3
5
8
10
H
H
18
Disubstituted Cyclohexanes:
Cis & trans Substituents:
Cis-
Trans-
1. 1,2 substitution:
2. 1,3 substitution:
3. 1,4 substitution:
Examples of disubstituted cyclohexanes and the equilibria for the chair conformers.
19
Bicyclic Compounds:
Compounds containing 2 joined rings.
Types: 1. Brigded
2. Fused EXAMPLES
3. Spirocyclic
Nomenclature:
a) For Bridged and Fused:
• Locate the bridgehead.
• Count the number of carbons in each bridge, excluding the bridgehead
carbons.
• Start with the prefix “bicyclo”
• Follow with the bridge count in decreasing order of carbons.
• List the name of the alkane matching the total number of carbons in the
bicyclic compound.
Example:
____________________________ _______________________________
b) For Spirocyclics:
• Start with the prefix “spiro”
• Count the carbons of the two tethered portions joined at one point, excluding the
spiro carbon.
• Follow with the tether count in decreasing order of carbons.
• List the name of the alkane matching the total number of carbons in the spiro
compound.
20
Example:
__________________________