lecture 6 - CHEM 231 Conformations of straight chain...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: CHEM 231 Conformations of straight chain alkanes and substituted alkanes LECTURE 6 LECTURE 6 1 Ground state energy of a molecule Each molecule has an inherent ground state energy. Any molecule naturally exists at its lowest energy unless an outside force acts on it. Any parameter that can be changed in organic chemistry can affect the ground state energy of a molecule Chapter 3 focuses on parameters that effect the energy of molecules Lecture 6 2 Gibbs free energy equation G = H TS G = RT ln K Lecture 6 3 Reading an energy diagram Free energy Progress of reaction Lecture 6 4 Understanding what applied energy does to the energy of a population of molecules At low T narrow distribution of energy few molecules have activation E At hi T broad distribution of energy and many molecules have activation E Lecture 6 5 Torsional strain Torsional strain is the repulsion caused by sets of bonding electrons being held too close together Lecture 6 6 Van der Waals strain Van der Waals strain is also known as steric strain. This type of strain is caused by repulsion caused by electron cloud overlap Lecture 6 7 Angle strain Angle strain is the strain caused by bonds being outside of normal hybridized angle limits Angle strain is a common type of strain in 3 and 4member rings Lecture 6 8 Definition of conformer A conformer is a molecular arrangement achieved solely through the rotations along bond axes. Lecture 6 9 Methane has only one conformer Any rotation of sigma bonds leads to a molecule that has an identical shape Methane Lecture 6 10 Ethane Here are a few ways to look at ethane H H H C H C H H H H H H H H H H H H H H Lecture 6 11 The 2 conformers of Ethane Lecture 6 12 Energy differences of the 2 conformers of Ethane Lecture 6 13 Butane Butane has more degrees of freedom than ethane or chloroethane and thus will have a complicated energy profile Since there is a difference between the methyl groups and hydrogen a new conformer comes about. Lecture 6 14 Butane and its conformations H H CH3 H H H3C H CH3 H H H3C CH3 H H eclipsed CH3 anti H gauche H H Lecture 6 15 The energetics of butane conformations Methane has only one conformer Lecture 6 16 The energetics of butane conformations With ball and stick we can sort of get the point but things don't look too strained! Lecture 6 The space filling model illustrates the torsional and van der Waals strains of eclipsed conformer 17 Ground state energies of cycloalkanes Compound name cyclopropane cyclobutane cyclopentane cyclohexane cycloheptane cyclooctane cyclononane cyclodecane cyclotetradecane # of CH2 groups 3 4 5 6 7 8 9 10 14 Ground state Energy per CH2 energy group (kJ/mol (kJ/mol) 2091 2721 3291 3920 4599 5267 5933 6587 9139 697 681 658 653 657 658 659 659 653 Lecture 6 18 Cyclopropanes and a new type of strain Angle strain is found in small rings Overlap is poor and angles are wrong leading to raised ground state energy levels Lecture 6 Highest electron density is not along the CC bond 19 Cyclopropanes Torsional strain comes from parallel bonds in the ring and bonds coming from the ring Lecture 6 20 Cis and trans groups influence strain differently. Lecture 6 21 Cyclopropanes simple nomenclature Lets practice cyclopropane nomenclature Draw the following 2 compounds: 1, 1, 2 trichlorocyclopropane; trans1methyl 2ethylcyclopropane Name the following compounds: Cl F F Lecture 6 Cl CH3 F 22 Cyclobutanes Adding one carbon relieves a lot of angle strain but we still end up with a ring that has little wiggle room Lecture 6 23 Substituted Cyclobutanes Cyclobutanes still suffer from angle strain and torsional strain. If there are groups cis to each other then there is also a great deal of van der Waals strain Lecture 6 24 Cyclobutanes Practice some cyclobutane nomenclature Draw: cis2fluoro1isopropylcyclobutane Secbutlylcyclobutane 1cyclobutyl3chloropentane Cl F F CH3 Br Br Lecture 6 25 Cyclopentanes Cyclopentane has one more carbon in the ring and the natural bond angle of flat cyclopentane is very similar to the sp hybridized Cyclopentane exists in the envelope type form to reduce torsional strain Cyclopentane is not a flat molecule Lecture 6 26 Why Cyclopentanes are common in nature G = H TS G = G + G + G For Cyclobutane: Angle strain is significant, torsional strain is significant, and there is very little entropy hard to make a small ring without putting in a bunch of energy For cyclopentane: Angle strain is minimal, torsional strain is minimal,still plenty of entropy Lecture 6 27 Some cyclopentane nomenclature Lets practice cyclopentane nomenclature: 1butyl2,2dimethylcyclopentane Cyclopentylcyclopropane F F Lecture 6 28 The 3 types of Bicyclic compounds Fused Bridged Spiricyclic Lecture 6 29 Fused bicyclic nomenclature Count the total number of carbons in the bicyclic frame work. Ignore any appending groups in this count Bicyclo heptane For naming purposes we now ignore the 2 carbons that are involved in the fuse we just count the other carbons in each of the rings 2 carbons Three carbons Zero carbons Bicyclo[3.2.0]heptane Lecture 6 30 Bridged bicyclic nomenclature Count the total number of carbons in the bicyclic frame work. Ignore any appending groups in this count Bicyclo octane For naming purposes we now ignore the 2 carbons that are involved in the fuse we just count the other carbons in each of the rings 2 carbons 2 carbons Bicyclo [2.2.2]octane 2 carbons Lecture 6 31 Count the total number of carbons in the spirocyclic frame work. Ignore any appending groups in this count spiro octane For naming purposes we now ignore the 2 carbons that are involved in the fuse we just count the other carbons in each of the rings since there is no possibility of a third ring in a cpirocyclic compound only 2 numbers are listed 4 carbons Spirocyclic compound nomenclature spiro[4.3]octane 3 carbons Lecture 6 32 ...
View Full Document

This note was uploaded on 10/06/2008 for the course CHEM 231 taught by Professor Kissling during the Spring '08 term at Binghamton.

Ask a homework question - tutors are online