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