Organic Flashcards
3 rules for resonance structures
Must have same molecular formula
Must have same total number of electrons
Must have same atoms bonded together
3 legal moves of resonance
π bond -> lone pair
lone pair -> π bond
π bond –> π bond
What are constitutional isomers?
Isomers with different connectives
What are meta, para and ortho structures?
Apply to a benzene ring with 2 groups attached
meta - groups are 2 carbons away
para - groups are on opposite sides
ortho - groups are next to each other
What are conformational isomers?
Can be converted into each other by rotation around a single bond
eg anti and syn structures
What are enantiomers?
Non-superimposable mirror images of each other
What are diastereoisomers?
Non-superimposable, not mirror images of each other
eg E/Z isomers, chiral molecules
What are configurational isomers?
Involves the breaking of bonds to convert between the 2
2 types: diastereoisomers, enantiomers
What are the 2 types of stereoisomers?
Configurational
Conformational
What is a conjugated system?
Has 2 or more C=C bonds alternating with C-C bonds
More conjugation leads to…
Smaller HOMO-LUMO gap
For a reaction to take place, molecules must:
Overcome their electronic repulsion by charge attraction and/or orbital overlap
Have orbitals of appropriate energy to react - a filled orbital on the nucleophile and an empty orbital on the electrophile
Approach each other such that these orbitals can overlap to form a bonding interaction
Nucleophiles typically…
Have a non-bonding lone pair of electrons (high energy HOMO)
eg ammonia, amines, water,
Generally, the higher the pKa of AH, the better A- is as a nucleophile
Nucleophiles donate electrons from…
Available, high-energy orbitals from: a lone pair a negative charge a double bond a σ bond to an electropositive atom
Electrophiles are…
Neutral or positively charged species with an empty atomic orbital or a low-energy antibonding orbital that can easily accept electrons
What makes a good electrophile?
Has a low-energy anti bonding orbital associated with an electronegative atom
e.g. Having a double or single bond to an electronegative atom
Electrophiles accept atoms into…
Empty low-energy orbitals represented by:
a positive charge representing an empty orbital
a neutral molecule with an empty p orbital
a double bond to an electronegative element
a single bond to an electronegative atom
What makes a stable anion?
Having the negative charge on an electronegative atom or by spreading the negative charge over several atoms
SN1 reaction
- Unimolecular
- Good LG needed
- Must be able to form a stable carbocation intermediate
- Nu independent
- Polar and protic solvent to stabilise ions
SN2 reaction
- Biomolecular
- Good LG needed
- Sterically unhindered substrates
- Good Nu needed
- Solvent: not protic, but polar to dissolve Nu
What molecules undergo an SN2 reaction?
CH3X > primary > secondary > tertiary (doesn’t react)
What molecules undergo an SN1 reaction?
tertiary > secondary > primary (doesn’t react) > CH3+ (doesn’t react)
What does the strength of an acid depend on?
The stability of the conjugate base
- the more stable the conjugate base, the stronger the acid
What makes a good LG?
Weak bases - lower pKa means a better leaving group
3 ways of making esters from alcohols
- acid anhydride + pyridine
- acid chloride + pyridine
- carboxylic acid + acid catalyst e.g. HCl
Order of reactivity of carbonyls
Acid chloride > Aldehyde > Ketone > Ester > Amide
The R/S isomer has the groups in order of decreasing priority in which direction?
R clockwise
S anticlockwise
Stereochemistry of SN1 and SN2 reaction
SN1 usually produces a racemic mixture
SN2 inverts the carbon being attacked
What are hard nucleophiles?
Small, charged, basic, low-energy HOMO, like to attack C=O, e.g. RO-, NH2-, MeLi
What are soft nucleophiles?
Large, neutral, not basic, high-energy HOMO, like to attack saturated carbon, e.g. RS-, I-, R3P
Which structure is more stable, staggered or eclipsed?
Staggered
What conditions favour elimination over substitution?
High temperatures
Nucleophiles that are strong bases and bulky
What conditions favour E2 reaction over E1?
High base concentration and strong bases favour E2
E1 reaction must…
Must form stable carbocation
Regio and stereo selectivity of E1 reaction
E isomers are favoured over Z
More stable alkenes are favoured
Regio and stereo selectivity of E2 reaction
E2 reactions favour more stable alkenes, but become more regioselective for the less substituted alkene with more hindered bases
What can never be a leaving group for E2?
OH-
Order of steps for E1, E2, E1cB
E1 - leaving group first, then deprotonation
E2 - deprotonation and leaving group simultaneous
E1cB - deprotonation first, then leaving group
Reducing agents
NaBH4 - will only reduce aldehydes and ketones
LiAlH4 - will reduce all carbonyls
Reagents for making an aldehyde from an alcohol
CrO3 (can over oxidise)
or PCC and DCM
or DMSO, (COCl)2, Et3N and DCM
Enolate systems tautomorise between…
Enol (alkene alcohol) and ketone
Markovnikov’s rule
For addition of HX across a double bond, the hydrogen will attach to the carbon with the most hydrogens on already
Stereochemistry of adding Br2 to an alkene
Only anti product is formed (Brs on opposite sides)
Brs sticking up either side, with R groups dashed and wedged
Stereochemistry for adding H2 to an alkene
Only syn product is formed (Hs on same side)
Stereochemistry of adding HBr to an alkene
An equal mixture of anti and syn products are formed
Alcohol + ketone –>
Acetal
where C=O was, turns into O-R (alcohol is ROH) going out both sides
Ketone + amine –>
Imine
C=O goes to C=NR (amine is RNH2)
Explain uses of enolates in organic chemistry
- Explain what they are
- Relative reactivity / stability
- How to make them
- Alkylations
- Condensations (aldol and claisen)
- Michael reactions
Rough pKa of carbonyls
Carboxylic acid - 5 Nitro - 9 Amide - 15 Alcohol - 17 Aldehyde - 17 Ketone - 20 Ester - 25 Nitrile - 25 Amine - 30 Alkane - 50
Definition of a chiral molecule
Is not superimposable on its mirror image (lack of plane of symmetry or centre of symmetry)
How to separate a mixture of entiomers
React with another chiral molecule to form diastereoisomers, which have different physical properties e.g. boiling point
