Organic Chemistry Flashcards
Isomer
Two or more compounds with the same formula but different arrangement of atoms and different properties
Constitutional Isomers
Different nature/sequence of bonds
Stereoisomers
Different arrangement of groups in space
Types of stereoisomers
Conformational and Configurational
Conformational Isomers
Differ by rotation about a single bond
Configurational Isomers
Interconversion requires breaking bonds
Note: Don’t need to have double bonds to have configurational isomerism (can be in ring)
Types of configurational isomers
Enantiomers and Diastereoisomers
Enantiomers
Non-superposable mirror images
Note: superposable means exact replica
Diastereoisomers
Not mirror images
Diastereomer (Z)
Z double bond = together
High priority groups on same side of C=C
Diastereomer (E)
E double bond = opposite
High priority groups on opposite sides of C=C
Diastereomer (cis)
cis = same side (3D) = Z = together
Diastereomer (trans)
trans = opposite side (3D) = E = opposite
Inductive Effect
Carbocation stability
Alkyl groups help to spread the positive charge a little along the bonds.
In tertiary - charge spread over 3 more carbon atoms
Hyperconjugation
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
Mass spectrometry
Determines molecular formula
Electron Impact (EI) Mass Spectrometry
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
Molecular ion
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.
Base peak
The most intense signal
High Resolution Mass Spectrometry (HRMS)
HRMS uses the most abundant isotope
Note: masses on the periodic table are weighted averages
NMR Spectroscopy
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.
What does a C-13 NMR spectrum tell us?
- no. of signals -> no. of C environments
2. position of signal -> type of environment
Chemical shift
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
Two sources of extra information in a H-1 spectrum
- The integral - tells you how many H’s in a given environment
- The multiplicity (splitting) - tells you how many neighbours an H has. The n+1 rule.
Infrared and UV/Vis spectroscopy
Identify functional groups and conjugation
SN2 reaction
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).
SN1 reaction
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.
E1 reaction
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.
E2 reaction
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
Chiral
An object is chiral if it is not superimposable upon its mirror image.
Stereogenic Centre
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.
Racemic Mixture
50:50 mixture of both enantiomers
The enantiomer is optically active when
plane polarised light is passed through a solution of one pure enantiomer of a compound, the plane of polarisation is rotated.
Enantiomer rotation in a clockwise direction is labelled
+
Enantiomer rotation in anti-clockwise direction is labelled
-
Racemic mixtures give an overall rotation of
zero
Absolute configuration
Stereogenic centre with high priority to low priority group in anticlockwise direction
S
Absolute configuration
Stereogenic centre with high priority to low priority group in clockwise direction
R
For n stereogenic centres there are how many possible isomers?
2^n
meso compound
Has chiral centres but is not a chiral compound. Has an internal plane of symmetry.
Fischer Projections
2D representations of 3D molecules
Horizontal lines: ‘stick out of page’
Vertical lines: ‘stick into page’
Grignard Reagents
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
