Structure of acetone
Structure of acetontrile
Structure of toluene
Diethyl ether structure
Dimethylsulfoxide (DMSO) structure
N,N-dimethylformamide (DMF) structure
General structure of an acetal
General structure of a carbonate
General structure of a nitrile
General structure of an ancyl chloride
What is a conjugated system?
A system with two or more C=C bonds alternating with C-C bonds
Briefly describe an SN1 reaction
- Substitution Nucleophillic Unimolecular
- Reaction passes through a carbocation intermediate
HA + H2O A- + H3O+
[A-][H3O+] / [HA]
- log (Ka)
pKa range of strong acids
-10 to 0
pKa range of weak acids
0 to 16
pKa range of very weak acids
16 to 50
pKa + log ( [HA] / [A-] )
What is a constitutional isomer?
Same molecular formula, different connectives
What is a stereoisomer?
Same molecular formula, same connectives
What are the two types of stereoisomers?
Configurational - 2 types
Conformational - Molecules can be converted into one another by rotation around a single bond
What are the two types of configurational isomers?
Enantiomers - Non-superposable mirror images
Diastereoisomers - Non-superposable, not mirror images
eg. E/Z isomerism
'One end stays the same, the other is mirrored (wedge and dash switch)'
What is a chiral molecule?
A molecule that has a non-superposable mirror image. Has none of the following symmetry elements: A mirror plane, a centre of inversion, a rotation-reflection axis
What is an achiral molecule?
A molecule that has a superposable mirror image
What is a prochiral molecule?
A molecule that can be made chiral by swapping/adding one group
Measurement of optical activity formula
observed angle of rotation (o) / [path length (dm) x concentration (g ml-1)]
How to decide if an isomer is E or Z
Find highest priority group on each carbon
- Higher atomic number
2 highest priority groups are on the same side is the Z isomer, when they are on the same side it is the E isomer
How can the configuration of 2 diastereoisomers (E/Z) be determined?
- They will differ in boiling point and the C13 and H1 NMR spectrums will differ, but it is hard to assign the configuration this way
- If crystals of either can be grown at low temperature, the crystal can be analysed by X-ray crystallography to determine the configuration
How can the confoguration of 2 diastereoisomers (anti/syn) be determined?
- They are difficult to distinguish or separate
- Might be able to use low temperature microwave spectroscopy
How to decide if an isomer is anti or syn
The syn isomer will have the 2 highest priority groups (or the 2 groups that have been added) on the same side, the anti isomer will have them on opposite sides
What physical properties will 2 constitutional isomers differ in?
They will have different melting points, and their C13 and H1 NMR spectrums will differ (give examples)
On benzene, where do the ortho, meta and para groups lie?
How can the configuration of 2 enantiomers be determined?
They can be distinguished by their ability in solution to rotate the plane of plane-polarised light
The actual configuration can be determined by X-ray crystalography of a single crystal sample of a pure enantiomers
More conjugation leads to...
Smaller HOMO-LUMO gap
ie absorbs lower energy
Factors determining the strength of the acid
- Most important: Stability of the conjugate base, a more stable conjugate base means a stronger acid
- Bond strength H-A, the easier the bond is broken, stronger the acid
- The solvent, the better the solvent is at stabilizing the ions formed, the stronger the acid
What makes a more stable conjugate base?
- If the negative charge is on a more electronegative atom, the conjugate base will be more stable
- Delocalization of the negative charge, the more spread out the charge is, the more stable the conjugate base
Explain which is more acidic: phenol or nitrophenol
- The molecules both react with water, leaving a negative charge on the O
- Nitrophenol has an NO2 group which is electron-withdrawing, stabilizing the conjugate base with inductive effects
- So nitrophenol is more acidic
Describe the structure of nitrophenol
HO - benzene - N+ = O
Briefly describe an SN1 reaction
- Leaving group goes first
- Carbocation is formed
- Nucleophile attacks next
Briefly describe an SN2 reaction
Nucleophile attacks at the same time as the leaving group goes
The rate of an SN2 reaction depends on...
The concentration of the substrate and the concentration of the nucelophile
What increases the strength of inductive effects?
More electronegative atoms on the molecule, and these atoms being closer to the ionisation site
Small pKa =
a strong acid
Explain how to draw R and S isomers
Rank the groups from highest to lowest priority
Place them as follows:
R - starting from the top bond, go clockwise (rectus) putting the lowest priority group at the back
S - starting from the wedge, go anticlockwise (sinister) putting the lowest priority group at the back
What is topicity?
It relates atoms, groups or faces to each other within molecueles
What does constitutionally heterotopic mean?
What does homotopic mean?
Atoms, groups or faces are related by an n-fold rotation axis, and the molecules are essentially the same
Eg. Gives the same molecule when Hs are replaced in turn, so the H atoms are homotopic
What does enantiotopic mean?
Constitutionally identcal atoms, groups or faces in molecules which are related by a mirror plane, inversion centre or improper rotation axis. Replacement of one pair of enantiotopic groups forms one of a pair of enantiomers
What does diastereotopic mean?
Constiutionally equivalent atoms, groups or faces of a molecule which are not symmetry related. Replacement of one or two diastereotopic atoms or groups results in the formation of one of a pair of diastereoisomers
The rate of an SN1 raction depends on...
- The carbon skeleton
- The leaving group
What substrates are likely to undergo SN1 mechanism?
- Nu's attack is hindered and a carbocation is readily formed
Allylic (double bond) (good)
Adj. lone pair
What substrates are likely to undergo SN2 mechanism?
- Nu can attack unhindered and a carbocation forms reluctantly
Allylic (double bond) (good)
What makes a good leaving group?
A weak base / a strong acid
eg I, Cl
OH is never a leaving group (unless protonated with a string acid)
What makes a good nucleophile?
- Nucleophillicity increases with charge density
- Nucleophillicity is the donation of an electron pair, the less electronegative the atom, the less 'tightly held' the electrons will be, so the better nucleophile
- In polar protic solvents, nucleophillicity increases going down the periodic table. The capacity for hydrogen bonding is highest at the top of the periodic table, less hydrogen bonding means the nucleophile is less hindered
- Less sterically hindered (less bulky) nucleophiles are better
Examples of polar protic solvents
Acetone, DMF, MeCN, DMSO
Why is OH- a good nucleophile?
It is a strong base
Characteristics of a hard nucleophile
- Low-energy HOMO
- Like to attack C=O
- Such as RO-, NH2-, MeLi
Characteristics of a soft nucleophile
- Not basic
- High-energy HOMO
- Like to attack saturated carbon
- Such as RS-, I-, R3P
Why is tBu a bad electrophile?
Gives steric hinderance to the approach of the Nu, raising the energy of the transition state structure, giving a very slow rate of reaction