Chapter 3 Organic Compounds: Alkanes and Their Stereochemistry.
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Transcript of Chapter 3 Organic Compounds: Alkanes and Their Stereochemistry.
Chapter 3Organic Compounds:
Alkanes and Their Stereochemistry
Alkanes have high activation energies for reaction, but are the backbone system for all organic compounds (by definition) and will be used here to introduce important ideas.
Alkane backbones will be used in the approach to naming organic compounds.
We will take an initial look at 3-D aspects of molecules
Why this Chapter?
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 and Br2 in the presence of light to replace H’s with Cl’s (not controlled here)
Chemical Properties of Alkanes
Monochlorinate Butane1) When butane is chlorinated, the C1 and C2
products are different.
2 structural isomers
Cl
Cl
1-chlorobutane 2-chlorobutane
*
•A chiral carbon•See CH 15
Mechanism of formationCl Cl
H H
H
Cl
2 Cl
+ Cl .+ HCl
.+ Cl
Both mirror images0ptically different
PlanerIntermediate
Boiling points and melting points increase as size of alkane increases
London Dispersion forces increase as molecule size increases, resulting in higher melting and boiling points
Physical Properties of Alkanes
We can represent an alkane in a brief form or in many types of extended forms
A condensed structure does not show bonds but lists atoms, such as CH3CH2CH2CH3 (butane) CH3(CH2)2CH3 (butane)
Structural formulas
Structures of Alkanes
A line structure does show bonds but doesn’t list carbon or hydrogen atoms, such as
Structural formulas
Line Structures of Alkanes
Stereochemistry concerned with the 3-D aspects of molecules
bonds are cylindrically symmetrical Rotation is possible around C-C bonds in open-
chain molecules
Conformations of Ethane
Different Conformations- Different arrangement of atoms resulting from bond rotation
Conformations can be represented in 2 ways:
Conformers
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
Torsional Strain
Energies Associated with Staggered or Eclipsed Conformations
The eclipsed conformer of propane has 3 interactions: two ethane-type H-H interactions, and one H-CH3 interaction
Conformations of Other Alkanes
Conformational situation is more complex for larger alkanes
Not all staggered conformations have same energy, and not all eclipsed conformations have same energy
Conformations of Other Alkanes
Anti conformation- methyl groups are 180˚ apart Gauche conformation- methyl groups are 60˚ apart
Which is the most energetically stable?
Conformations of Butane
Steric strain- repulsive interaction occurring between atoms that are forced closer together than their atomic radii allow
Steric Strain
Conformational Energy Costs
Conformational Studies x-ray and IR
• Information on the most stable conformational isomer (A Conformer) comes from single-crystal X-ray diffraction studies.
• IR spectroscopy is ordinarily used to measure conformer ratios. For the axial and equatorial conformer of bromocyclohexane, νCBr differs by almost 50 cm−1.[1]
[1] Eliel, E. L.; Wilen, S. H.; Mander, L. N. "Stereochemistry Of Organic Compounds", J. Wiley and Sons, 1994. ISBN 0-471-01670-5.
Conformational NMR Studies
The dynamics of conformational (and other kinds of) isomerism can be monitored by NMR spectroscopy at varying temperatures. The technique applies to barriers of 32 to 60 kj/mol, and species exhibiting such dynamics are often called "fluxional".
Extremely low temperature NMR studies can “see” even lower barriers but special instruments and solvents must be used.
Yes – There is math involved! But, it is pretty straight forward. Take Physical Organic Chemistry next year.
Conformational NamingNaming alkane conformers according to the Klyne-Prelog System for specifying angles (called either torsional or dihedral angles) between substituents around a single bond:
a torsion angle of ±60° is called gauche [8]
a torsion angle between 0° and ± 90° is called syn (s)a torsion angle between ± 90° and 180° is called anti (a)a torsion angle between 30° and 150° or between –30° and –150° is called clinala torsion angle between 0° and ± 30° or ± 150° and 180° is called periplanar (p)a torsion angle between 0° and ± 30° is called synperiplanar or syn- or cis-conformation (sp)a torsion angle between 30° to 90° and –30° to –90° is called synclinal or gauche or skew (sc)[9]
a torsion angle between 90° and 150° or –90° and –150° is called anticlinal (ac)a torsion angle between ± 150° and 180° is called antiperiplanar or anti or trans (ap).
Conformational Naming
Conformation ApplicationsReaction rates can be highly dependent on the conformation of the reactants. This theme is especially well elucidated in organic chemistry. One example is provided by studying elimination reaction mechanisms, which involve the simultaneous removal of a proton and a leaving group from vicinal positions under the influence of a base. The mechanism requires that the departing atoms or groups follow antiparallel trajectories.
Constitutional Isomers
Breaking Bonds
Just won’t happen at room temperature
Constitutional Isomers
Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)
The formula for an alkane with no rings in it must be CnH2n+2
Alkanes are saturated with hydrogen (no more can be added
They are also called aliphatic compounds
Alkanes and Alkane Isomers
CH4 = methane, C2H6 = ethane, C3H8= propane 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
Straight-chain or normal alkanes
Branched-chain alkanes
Alkane Isomers
Isomers that differ in how their atoms are bonded are called constitutional isomers
They must have the same molecular formula to be isomers
Constitutional Isomers
Draw as many compounds as you can that are isomers with the formula, C4H10:
Let’s Work a Problem
AnswerThe safest approach to answer this question would be to draw out all straight-chain isomers, then proceed next to the simplest branched structures and so on.
Draw as many compounds as you can that are isomers with the formula, C4H10O:
Now add an oxygen atom
Answer
1-butanol 2-butanol isobutyl alcohol tert-butyl alcohol
Naming
Naming Straight Chain Alkanes
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
Alkyl Groups
Alkyl Groups (Continued)
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)
Types of Alkyl Groups
* There is no 4˚ hydrogen…Why or why not? Let’s talk about this…
Alkyl Groups (Continued)
Compounds are given systematic names by a process that uses
Follows specific rules Find parent hydrocarbon chain
Naming Alkanes
Naming Alkanes (Continued)
Carbons in that main chain are numbered in sequence
Substituents are identified and numbered
Naming Alkanes (Continued)
Write compound name is single word Name a complex substituents as though it were a
compound itself See specific examples in text
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
Functional Groups
Functional group - A collection of atoms which have a lower activation energy towards 2 electron mechanistic reactions compared to alkane non-functional parts of the molecule and react in a similar manner regardless of where in a molecule they are placed.
Functional Groups
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
Functional Groups with Multiple Carbon–Carbon Bonds
Functional Groups with Carbon Singly Bonded to an Electronegative Atom
Functional Groups with a Carbon–Oxygen Double Bond (Carbonyl Groups)
Survey of Functional Groups
Survey of Functional Groups