Organic Chemistry Pt 1 - Week 7-9 Flashcards
Molecular formula defintion:
Type and number of each element present
What 4 things must be specified when naming organic molecules?
- Carbon chain length
- Functional group class
- Any substituents present and where they are
on the molecule - Put everything together in alphabetical order
What is the priority order for functional groups?
Carboxylic acid — top priority
Ester
Acyl halide
Amide
Nitrile
Aldehyde
Ketone
Alcohol
Thiol
Amine
Ether
Alkene
Alkyne
Alkyl halide
Draw out 4-bromo-5-hydroxy-6-methylhepta-2-one
Longest chain is 7 –> hepta-
Principal group is C=O –> one ending
OH –> hydroxy
Br –> bromo
CH3 –> methyl
What are the 4 key physical properties of hydrocarbons?
States of matter: C1 - C4 gasses
C5 - C17 liquids
C17+ solids
Less dense than water
Molar mass increases with chain length
Boiling point - increases with chain length
- decreases with branching
What 4 factors are affected by increasing chain length?
- Increases bp
- Increases strength of van der Waals forces
- Increases Mr
- More likely to be liquid or solid at RT
Which linear molecules can contort into different conformations?
Alkanes can contort into different conformations –> due to rotation of bonds
Alkenes are frozen in a particular conformation
What are the two possible conformations of cyclic molecules and how do conformations occur?
- Chair
- Boat
Conformations have an energy difference so a particular conformation dominates at rt.
What are constitutional isomers (structural isomers) ?
- Chain isomers
- Position isomers
- Functional group isomers
How can you identify between geometric isomers?
Use E/Z or cis/trans system.
What are the 3 rules for assigning highest priority groups?
Higher atomic number = higher priority
If a group has to atoms of the same type bonded to the alkene follow the chain until a difference in priority occurs.
Bonds of a double bond are counted additively e.g. worth 2 single bonds same in an alkyne (‘worth’ 3 single bonds)
How does the E/Z system work?
Assign priority to each group on a C in the C=C
High priority on same side = Z
High priority on opposite side = E
How does the Cis/ trans system work?
Need each C to have one group in common
Same side = Cis
Opposite side = Trans
What occurs with the combustion of hydrocarbons?
Hydrocarbons combust to form carbon dioxide and water
Incomplete combustion forms carbon monoxide and water
What is a nucleophile?
electron donor, has an excess of electrons
Loves nuclei
What is an electrophile
Neutron donor, has and excess of neutrons
Loves electrons
What reaction do alkenes undergo?
React via electrophilic addition mechanism - due to nucleophilic C=C
Addition of X2 to form a dihaloalkane
Addition of H2 to form an alkane
What is markovnikov’s rule?
The major product forms from the addition of the (electrophile) H atom to the C with the most H atoms.
What conditions are required for the hydration of alkenes?
A small amount of concentrated acid as a catalyst - H3PO4 or H2SO4
What are the special characteristics of arenes?
Arenes have a special type of delocalisation that imparts special stability.
Alternating double and single bonds are conjugated with delocalised pi-electrons.
What are the 3 criteria for the Hukel criteria for aromaticity?
- The compound must be cyclic
- The atoms in the ring must be sp2
hybridised (have a free p-orbital) - The sum of the electrons contained in the p
orbitals must correspond to 4n + 2
electrons n= 0, 1, 2, 3 (2e-, 6e-, 10e-, 14e-)
What are haloalkanes?
Alkanes containing a halogen atom (F, Cl, Br, I)
Polar molecules have a permanent dipole at the carbon halogen bond
What are 3 haloalkanes that are commonly used as solvents?
- 1,2- dichloroethane
- dichloromethane (DCM)
- Chloroform
What are 4 reasons why haloalkanes are commonly used as solvents?
- have a moderate polarity compared to water
- able to dissolve most organic compounds
- Lower bp means easier to evaporate off
- DCM and chloroform are non-flammable
What are the physical properties of halogenated alkanes? State, solubility, density, intermolecular forces and boiling point?
State - Usually liquid (or solid) at RT
(exceptions are CH3Cl and CH3Br)
Solubility - haloalkanes generally soluble with
organic solvent
- Haloalkanes insoluble in water
Density - More dense than water
Intermolecular forces - Dipole-Dipole, Van der
Waal
Boiling point - higher than hydrocarbons of
similar length
- type of halogen atom increases
as you go from Cl -> Br -> I
- number of halogen atoms
Why are halogens often used in drugs?
- slow down metabolic processes of the drug
- based on natural products
- improve bioavailability
- improve activity of drug relative to non-
halogenated analogue - Decreases lipophilicity -> less drug gets lokes
in fat stores
What is the difference between protic and aprotic solvents?
Polar protic solvents have a permanent dipole and can make H-bonds (usually have an O-H bond)
Polar protic solvents stabilise ions making them less reactive
Polar aprotic solvents have a permanent dipole but cannot make H- bonds
Ions are more reactive in polar aprotic solvents.
What are the two competing reaction pathways for haloalkanes?
Substitution pathway
Elimination pathway
What is nucleophilic substitution?
The nucleophile substitutes for a good leaving group on the molecule.
- Accesses new functional group classes
- Uses a variety of nucleophiles
What are the two types of nucleophilic substitution?
First order nucleophilic substitution (SN1)
Second order nucleophilic substitution (SN2)
What 3 things makes something a good nucleophile?
- Full negative charge
- Electronegative atom
- Small (not bulky)
Name 3 good nucleophiles and 3 weak nucleophiles:
Good = O-H, R-O, H3C-O, X
Weak = H2O, R-OH, R-SH, NH3
What makes something a weak nucleophile?
Has a free lone pair with a partial negative charge rather than a full negative charge.
What are the key steps in the SN2 general mechanism?
- Backside Nucleophile attack
- intermediate forms
- inversion of stereochemistry
What are the steps in the SN1 mechanism?
- Leaving group leaves
- Intermediate forms
- Nucleophilic attack
- If chiral racemic mixture produced
What pathway do methyl and primary haloalkanes always react via?
SN2 pathway
What pathway do tertiary haloalkanes always react via?
a SN1 pathway
What pathway do secondary haloalkanes react by?
a mixture of SN1 and SN2
What solvent is best for SN1 pathways?
Polar protic solvents
What solvents are best for SN2 pathways?
Polar aprotic solvents.
Which pathways do good nucleophiles favour?
SN2 pathways
Which pathway do weak nucleophiles favour?
SN1 pathway
What do elimination reactions do to haloalkanes?
Convert haloalkanes into alkenes
Base attracts a beta H and leaving group leaves.
What are the steps to an E2 general mechanism?
Base is attracted to a hydrogen atom.
Transition state formed
Beta hydrogen elimination kicks off leaving group
What are the steps to an E1 general mechanism?
Leaving group leaves
Beta hydrogen elimination quenches positive carbon
Product forms + (Base + H group)
What is the trend with alkene stability?
the more C groups bound to C=C the more stable it is
Which out of E - alkenes and Z - alkenes are the most stable?
E-alkenes are more stable than Z-alkenes
What is Zaitsev’s rule?
The major product formed will be the most stable alkene.
What pathway do primary haloalkanes undergo?
E2 pathway only
What pathway do secondary and tertiary haloalkanes undergo?
Either E1 or E2 pathway
Which solvents are best for E1 pathway?
Polar protic solvents
Which solvents are best for E2 pathway?
Polar aprotic solvents
What bases are favoured for E2 pathway?
Strong bases (preferentially hindered bases)
What bases are favoured for E1 pathway?
Weak bases
What are sterioisomers?
Isomers with the same molecular and structure, just a different 3D arrangement of atoms.
Requires a molecule with at least one stereocentre
What is an optical isomer?
Molecules with the same molecular and structural formula but are not superimposable.
- Need at least one sp3 hybridised C atom
bound to 4 different groups.
How do you assign R/S configuration?
Rank the 4 groups 1 - highest, 4 - lowest. H is always 4, lowest priority.
Rotate the molecule so the lowest priority is in the back, draw a circle going 1 -> 2 -> 3
What configuration does a clockwise molecule show?
R configuration
What configuration does an anticlockwise molecule show?
S configuration
How can you work out the maximum number of stereoisomers?
= 2^n n = number of stereocentres
What are enantiomers?
a pair of molecules that are mirror images because all chiral carbons in the molecule have opposite configurations
What are diastereomers?
Not mirror images
What are the 2 factors necessary for a molecule to be chiral?
- at least two chiral C’s
- at least one chiral C where the configuration
is not switched
What is a racemic mixture?
- a mixture containing equal amounts of both
enantiomers - specific rotation is zero
What are arenes?
- Cyclic aromatic molecules
- All atoms in the ring are sp2 hybridised
- P- orbitals contain 4n + 2 electrons ( 2e-, 6e-,
10 e-, 14 e-, … )
What naming system is used for di - substituted benzene derivatives?
Ortho , Meta, Para
One More Place
What are the 6 reaction mechanism that benzene can undergo?
- Electrophilic aromatic substitution
- Halogenation
- Nitration
- Sulfonation
- Friedel - Crafts Acylation
- Friedel - Crafts Alkylation
What are the 3 key stages of electrophilic aromatic substitution?
- Activation of electrophile
- Nucleophilic attack and loss of aromaticity
- Deprotonation and restoration of
aromaticity
mechanism preserves aromaticity and special stability it gives
