CHEM 1012 - Lecture notes 1-31 PDF

Preview

Summary

notes for chem 1012...

Description

CHEM 1012 Lecture One Revision of organic chemistry concepts

Lecture Two Classifying reactions Curly arrows Chemical reactions are all about electrons. Classifying reactions 1. Addition  groups added 2. Substitution  one group replaces another 3. Elimination  groups taken away 4. Oxidation-reduction  loss and gain of electrons 5. Acid-base  loss and gain of protons (H+) Nucleophile  nucleus loving  negative charge  attracted to positive charges Electrophile  electron loving  positive charge  attracted to negative charges Mechanisms A mechanism shows the exact sequence of steps that takes place in a reaction. - Represents using curly arrows to show how electron move

Reaction type 1 – acid base reactions An acid donates a proton, base accepts a proton.

The degree of acidity is related to the stability of the anion. Reaction type 2 – substitutions One group takes the place of another

Reaction type 3 – additions Groups are added to the organic starting material.

Reaction type 4 – eliminations Groups are taken away from the organic starting material.

Reaction type 5 –oxidation/reduction An oxidation will result in a decrease in the number of C-H bonds, or an increase in the number of C-O bonds.

A reduction will increase in the number of C-H bonds, or a decrease in the number of C-O bonds.

Lecture Three

Addition to alkenes Carbocations The C=C bond in alkenes  and  bonds are different -  bond is weaker and more reactive than  bond The symmetry of  bond means that rotation around the C=C bond cannot occur without breaking the  bond.

Electrophiles are attracted to the  bond. Addition occurs with overall replacement of a  bond with a  bond: energetically favourable.

What’s the mechanism? Two steps - Via an unstable intermediate

Carbocation The intermediate in the reaction is a carbocation. - Carbocations have 6 electrons and a positive charge - All carbocations are unstable but not all to the same extent.

More substituted carbocations are more stable due to inductive effects. and hypoconjugation. Product isomers Some alkenes can give mixtures of products. However, more of one than the other. If the carbocation is easier to form, more stable, the reaction goes faster. Product selectivity  Markovinikov’s rule

Lecture Four Substitution reactions: SN1 and SN2

Organohalides in substitutions

SN2 Reaction rate proportional to concentrations of both electrophile and nucleophile - both (2) of the reagents

nucleophile

electrophile

product

Common nucleophiles

SN1 Reaction rate proportional to concentrations of the organohalide only.

Formation of a carbocation first – rate limiting step. The second step is fast. SN1 vs SN2

Decide whether it is SN1 vs SN2 from the Carboncation stability and measuring the rate.

Lecture Five Elimination reactions: E1 and E2 Carbocation intermediates can also lead to elimination products. ELIMINATION FIRST ORDER The reaction rate depends only on starting material concentration  E1 (elimination first order). Carbocation stability We usually only see E1 for very stable tertiary carbocations. Zaitsev’s rule: If there’s a choice of double bond in E1. The more substituted double bond forms. ELIMINATION SECOND ORDER The reaction rate depends on the concentration of both reagents  E2 (elimination second order)

Measured rate: proportional to concentrations of bromopropane and potassium hydroxide. Selectivities in E2

Lecture Six Overview of mechanisms 1 Nucleophiles are attracted to electrophiles.

A curly arrow indicates electron pair movement.

Alkyl groups help to spread the positive charge a little along the bonds – an inductive effect.

Selectivity depends on the size of the base and the alkyl.

Lecture Seven Structure elucidation Mass Spectrometer Structure elucidation There are number of stages involved with determining the molecular structure of an unknown compound: 1. Isolate and purify it  precipitation, recrystallisation, chromatography 2. Determine molecular formula  mass spectrometry 3. Identify functional groups and conjugation  infrared and UV spectroscopies 4. Assemble structural formula  NMR spectroscopy As we go down, it is increasingly useful (more information). Empirical formula For organic compounds  combustion analysis - React with O2 and measure how much CO2 and H2O released - Can’t measure O easily, so oxygen assumed to make up difference to 100%  work out percentage composition Molecular formula - Limited use as their molecular masses could be different - e.g. C6H126  CH2O

Mass Spectrometer

-

-

measures the mass of molecules molecules are ionised by impact with electron in high vacuum  turned into ions  some ions fragment to form smaller ions  ions are accelerated through an electric field small molecules are deflected the most ions are separated according to m/z (mass/charge)  z is almost always 1 beams of ions directed into magnetic field, causing ions to bend detector measures how far ions have bent, so can work out m

Molecular and daughter ions - the heaviest ion possible is the molecular ion results from loss of one electron from the parent molecule an electron is so light that the mass of the molecular ion is same as parent molecule -

fragmentation may occur to give daughter ions these can help us determine the structure of the molecule

A typical mass spectrum has a molecular ion and many daughter ions  most intense signal is called the base peak  intensity is set to 100%. Mass spectrometry to help determine structure has declined to recent years  useful to get molecular ions not fragments. The most common process is electrospray.

Electrospray ionisation Formation of aerosol, includes mild acid/buffer, so ions formed as MH+ - ions enter mass analyser as before - very gentle  can even use to analyse proteins Influence of isotopes Each ion recorded in the mass spectrum registers the specific isotopes of the elements present. - Many elements have only one isotope of high abundance - Some elements have more than one abundant isotope e.g. sample of bromobenzene contains near equal amounts  two molecular ion peaks. High resolution mass spec High resolution mass spectrometry uses the most abundant isotopes. The numbers we use to calculate the mass are usually approximations  exact mass of an atom is not a round number  different molecule values.

Lecture Eight 13C NMR spectroscopy Nuclear magnetic resonance Spectroscopy

Involves absorption of part of the electromagnetic spectrum Triggers a change of energy level occupancy within the molecule The light absorbed is related to the process involved. Energy absorbed  signal in spectrum  structural information -

Nuclear magnetic resonance Uses energy in the radio frequency region Involves ‘flipping’ the spin of a nucleus in a magnetic field - Used in analysis and structure determination (NMR) - Used to image soft tissue in animals (MRI) Spin-active nuclei Principles of NMR spectroscopy Many nuclei are NMR active e.g. 1H, 13C, 31P, 19F Each resonates at particular frequency in given magnetic field Provides information on structural connectivity

Atomic nuclei behave as they are spinning about an axis  act like ‘mini magnets’. There is an energy difference between the two alignments Radio waves make low energy spin ‘flip’ to high energy status.

13C NMR spectroscopy Exact magnetic field seen by each nucleus depends on: - The externally applied magnetic field - The electronic environment surrounding the nucleus The extent to which electrons ‘shield’ a nucleus depends on: - The bonding to that atom (single vs double vs triple) - The number of electrons associated with surrounding atoms (i.e. the types of atoms attached: C vs H vs O vs Cl vs N).

Carbon environments The number of signals in an NMR spectrum reflects the number of carbon environments in the molecule Within a molecule two carbon environments are identical only when the bonding and neighbours are identical.

Chemical shift The position of a signal is called its chemical shift. - Referenced to as TMS (set as 0.00ppm)

Lecture Nine 1

H NMR spectroscopy Integrals and splitting

Quiz wk 3  IR spectroscopy visualises bond vibrations within a molecule, so reveals the presence (or absence) of particular functional groups with the presence (or absence) of key signals in the IR spectrum. 

UV/Vis spectroscopy visualises electronic transitions within a molecule, so reveals the presence (or absence) of conjugation. (Conjugation lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which is the gap across which an electron is excited by UV/Visible light.)



1H NMR spectroscopy shows the connectivity (bonding) and shape (stereochemistry) within a molecule. Each different type of hydrogen atom gives rise to a different signal in the spectrum, and the position and shape of this signal depend on the exact chemical environment around that hydrogen atom (i.e. the other atoms and bonds nearby).



Mass spectrometry measures the mass:charge ratio (m/z ratio) of individual ions, including the molecular ion (i.e. the ionised form of the molecule). This effectively allows us to ''weigh'' individual molecules, giving molecular weight information from which we can deduce a molecular formula.

Any compound that includes an C=O bond would be expected to give a signal in this region of the IR spectrum. 1650-1800 wavenumbers. Any compound that includes an N-H or O-H bond would be expected to give a signal in this region of the IR spectrum. 3000-3500 wavenumbers. Conjugation is an alternating sequence of single and multiple bonds (such that three or more adjacent atoms all bear an unhybridised p orbital).

Lecture Ten Acid and base properties Elimination Oxidation and reduction Alcohols have high boiling point, and are soluble in water. Alcohols are not very acidic or basic. Oxidation of organic molecules increased their O content or decreases their H content. Primary alcohol  aldehyde/carboxylic acid Secondary alcohol  ketone Tertiary alcohol  cannot oxidise Amines are basic and nucleophilic Good at H-bonding.

Lecture Eleven Addition to C=Os Oxidation and reduction

Lecture Twelve Acyl chlorides, esters, amides Nucleophilic acyl substitution...