Organic Chemistry

Question 1

Isomer

Answer 1

Two or more compounds with the same formula but different arrangement of atoms and different properties

Question 2

Constitutional Isomers

Answer 2

Different nature/sequence of bonds

Question 3

Stereoisomers

Answer 3

Different arrangement of groups in space

Question 4

Types of stereoisomers

Answer 4

Conformational and Configurational

Question 5

Conformational Isomers

Answer 5

Differ by rotation about a single bond

Question 6

Configurational Isomers

Answer 6

Interconversion requires breaking bonds

Note: Don’t need to have double bonds to have configurational isomerism (can be in ring)

Question 7

Types of configurational isomers

Answer 7

Enantiomers and Diastereoisomers

Question 8

Enantiomers

Answer 8

Non-superposable mirror images

Note: superposable means exact replica

Question 9

Diastereoisomers

Answer 9

Not mirror images

Question 10

Diastereomer (Z)

Answer 10

Z double bond = together

High priority groups on same side of C=C

Question 11

Diastereomer (E)

Answer 11

E double bond = opposite

High priority groups on opposite sides of C=C

Question 12

Diastereomer (cis)

Answer 12

cis = same side (3D) = Z = together

Question 13

Diastereomer (trans)

Answer 13

trans = opposite side (3D) = E = opposite

Question 14

Inductive Effect

Answer 14

Carbocation stability
Alkyl groups help to spread the positive charge a little along the bonds.
In tertiary - charge spread over 3 more carbon atoms

Question 15

Hyperconjugation

Answer 15

Carbocation stability
Adjacent C-H bonds donate a little electron density through space.
In tertiary - vacant p orbital receives partial donation of electrons from 9 possible C-H bonds

Question 16

Mass spectrometry

Answer 16

Determines molecular formula

Question 17

Electron Impact (EI) Mass Spectrometry

Answer 17

Measures mass of molecules

• molecules are ionised by impact with electrons
• some ions fragment to form smaller ions (daughter ions)
• ions accelerated through electric field
• ions directed into magnetic field and separate according to m/z (mass/charge)
• detector measures how far ions have bent, so can work out m

Question 18

Molecular ion

Answer 18

The heaviest ion possible.
Results from loss of one electron and is so light that the mass of the molecular ion is the same as parent molecule. Fragmentation may occur to give daughter ions.

Question 19

Base peak

Answer 19

The most intense signal

Question 20

High Resolution Mass Spectrometry (HRMS)

Answer 20

HRMS uses the most abundant isotope

Note: masses on the periodic table are weighted averages

Question 21

NMR Spectroscopy

Answer 21

Provides information on structural connectivity.
Energy absorbed -> signal in spectrum -> structural info

Many nuclei are NMR active: H-1, C-13, P-31, F-19. Each resonates at particular frequency in given magnetic field.

Question 22

What does a C-13 NMR spectrum tell us?

Answer 22

1. no. of signals -> no. of C environments

2. position of signal -> type of environment

Question 23

Chemical shift

Answer 23

The frequency signal between a signal arising from a sample and the TMS signal is called the chemical shift (the position of a signal).

Note: electronegative atoms deshield a C nucleus and shifts downfield (more left), C=C or aromatic ring also shift downfield

Question 24

Two sources of extra information in a H-1 spectrum

Answer 24

1. The integral - tells you how many H’s in a given environment
2. The multiplicity (splitting) - tells you how many neighbours an H has. The n+1 rule.

Question 25

Infrared and UV/Vis spectroscopy

Answer 25

Identify functional groups and conjugation

Question 26

SN2 reaction

Answer 26

Substitution Nucleophilic Second Order Direct attack of nucleophile. SN2 occurs in one step and BOTH nucleophile and substrate are involved in the rate determining step. The big barrier is steric hindrance. Rate of reaction proceeds from primary (fastest) > secondary > tertirary (slowest).

Question 27

SN1 reaction

Answer 27

Substitution Nucleophilic First Order Carbocation intermediate. Reaction proceeds stepwise. Involves the formation of a carbocation first (the rate limiting step) and is then attacked by a nucleophile (second step is fast). So reaction rate proportional to concentrations of organohalide only. Tertiary > secondary > primary.

Question 28

E1 reaction

Answer 28

Elimination First Order Rate depends only on the substrate, since the rate limiting step is the formation of a carbocation. A strong base is not required since there is no leaving group that needs to be displaced. Zaitsev's rule: the more substituted double bond forms.

Question 29

E2 reaction

Answer 29

Elimination Second Order Rate depends on both substrate and base. A strong base is required that will allow for displacement of a polar leaving group. (Pile electrons on an atom that can accept) Zaitsev's rule: the more substituted double bond forms

Question 30

Chiral

Answer 30

An object is chiral if it is not superimposable upon its mirror image.

Question 31

Stereogenic Centre

Answer 31

Also known as a chiral centre. An atom which has different groups bound to it in such a manner that its mirror image is non-superimposable.

Question 32

Racemic Mixture

Answer 32

50:50 mixture of both enantiomers

Question 33

The enantiomer is optically active when

Answer 33

plane polarised light is passed through a solution of one pure enantiomer of a compound, the plane of polarisation is rotated.

Question 34

Enantiomer rotation in a clockwise direction is labelled

Answer 34

+

Question 35

Enantiomer rotation in anti-clockwise direction is labelled

Answer 35

-

Question 36

Racemic mixtures give an overall rotation of

Answer 36

zero

Question 37

Absolute configuration | Stereogenic centre with high priority to low priority group in anticlockwise direction

Answer 37

S

Question 38

Absolute configuration | Stereogenic centre with high priority to low priority group in clockwise direction

Answer 38

R

Question 39

For n stereogenic centres there are how many possible isomers?

Answer 39

2^n

Question 40

meso compound

Answer 40

Has chiral centres but is not a chiral compound. Has an internal plane of symmetry.

Question 41

Fischer Projections

Answer 41

2D representations of 3D molecules Horizontal lines: 'stick out of page' Vertical lines: 'stick into page'

Question 42

Grignard Reagents

Answer 42

Electron-rich carbon can bond a little to an electropositive atom, like a metal. E.g. Mg (which is itself bonded to a halogen like Br): R-MgBr