Block 1 - Foundations Flashcards
Heteroatoms
Any atom not C or H
Second and third bond strength relative to first bond
Second bond (and third) must be weaker than the first
Covalent bonds form when…
… atomic orbitals overlap to form molecular orbits
Atomic orbital overlaps can be…
- Good (end-on), i.e. sigma bonds (σ-bonds)
- Weak (side-on), i.e. pi bonds (π-bonds aka DBEs)
Pi bonds weaker than sigma bonds
Single, double, triple bonds (σ or π)
Single: 1 sigma bond –> inert
Double: 1 sigma bond, 1 pi bond –> reactive
Triple: 1 sigma bond, 2 pi bonds
Hybridisation
The mixing of atomic orbitals belonging to the same atom, but having slightly different energies so a redistribution of energy takes place between them –> results in the formation of new orbitals of equal energies and identical shape
Rotation around double and triple bonds - π bonds
Rotation about double and triple bonds would require breaking a π-bond –> incurs significant energy penalty
C bonded to 4 atoms (single bonds)
Geometry: Tetrahedral (109.5°)
Hybridisation: sp3
C bonded to 3 atoms (two single bonds, one double bond)
Geometry: Trigonal planar (120°)
Hybridisation: sp2
C bonded to 2 atoms (one single bond, one triple bond OR two double bonds)
Geometry: Linear (180°)
Hybridisation: sp
Periodic table: First 20 elements
H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K, Ca
Periodic trends: Electronegativity, ionisation energy, atomic radius
Electronegativity: Increases up and right
Ionisation energy: Increases up and right
Atomic radius: Decreases up and right
Isomers
Compounds with same molecular formula, but different arrangement in space
Constitutional isomers
Different sequence of bonds
Double bond equivalents (DBEs) = ½ (2n4 + n3 - n1 + 2)
where…
DBE = ½ (2n4 + n3 - n1 + 2)
where n4 = C, n3 = N, n1 = H or halogens
O doesn’t appear in DBE formula
Stereoisomers
Different arrangement of groups in space
Conformational isomers
Differ by rotation about a single bond
No bond breaking required
Configurational isomers
Interconversion requires breaking bonds
Enantiomers
Non-superimposable mirror images
Stereogenic centre (C)
Chiral
Diastereomers
Not mirror images
Non-superimposable
Types of isomers
Isomers –> Constitutional and Stereoisomers
Stereoisomers –> Conformational and Configurational
Configurational –> Enantiomers and Diastereomers
Define conformer
The conformation of a molecule; a particular shape it adopts as a result of rotation about bonds
Conformational isomers - types of projection
Sawhorse projection (views molecules from an oblique angle) Newman projection (views molecules end-on)
Interconversion of conformers
Staggered —rotate 60°—> Eclipsed
Staggered conformers vs Eclipsed conformers
Staggered: atoms fit ‘between’ other atoms, most stable
Eclipsed: atoms fit directly ‘behind’ other atoms, least stable
Types of staggered and eclipse conformers
Staggered; anti-conformer: most stable
Eclipsed
Staggered; gauche-conformer
Eclipsed; syn-conformer: least stable
Cyclohexane conformers
Bond 109° preferred –> not flat/planar
Boat
Chair
Chair conformers: types of H atoms
Axial: pointing vertically up or down
Equitorial: point in other directions
Lowest energy conformation
Chair conformer
Ring flip
Converting from ‘Chair A’ to ‘Chair B’
Where all axial groups become equatorial groups and vice versa
Generally, the favoured conformer for substituted cyclohexanes have the largest atom(s)/group(s) in the ______
Equatorial
CIP rules
Determines whether E or Z configuration
Higher atomic number = higher priority = ranks higher
E vs Z configuration
Divide horizontally (i.e. like --------) E-configuration: two higher priority groups are on opposite sides of double bond Z-configuration: two higher priority groups are on same side of double bond
Stereogenic centre
Asymmetric carbon
Achiral
Where sp3 C has two identical atoms/groups attached so molecule is superimposable
Chiral
Where sp3 C has different atoms/groups attached so molecule is non-superimposable on its mirror image
Racemic mixture
1:1 ratio (equal amounts) of enantiomers
Rotates light equally in both directions –> overall rotation is zero
R vs S configuration
R: priority of highest substituents decreases in a clockwise direction
S: priority of highest substituents decreases in an anti-clockwise direction
Enantiomers, identical/meso and diastereomers
Enantiomers: mirror images, non-superimposable
Identical/meso: mirror images, superimposable
Diastereomers: not mirror images, non-superimposable
If octane is attached to something, it’s called ____
Stemane
CH3O name
Methoxy
Naming cycloalkanes
Add ‘cyclo’ to front of name, e.g. cyclopentane
Double bonds are more readily polarised due to…
The relative mobility of pi electrons, i.e. C=O is more polar than C-O bond
Branching and bpt
IF strongest when molecules are in close proximity, so decreased branching = increased bpt as molecules can stack closer together
Similar vs different molecular weights - bpt
Similar M: more polar functional groups = higher bpt
Different M: difference in temporary dipole-dipole interactions that has greatest effect on bpt
Polarity and water solubility
More polar = more water soluble (hydrophilic)
Less polar = more lipid soluble (lipophilic)
Polar stationary phase: Polar vs non-polar solutes
Polar solutes are more attracted to the polar stationary phase and move slowly
Non-polar solutes are less attracted to the polar surface of stationary phase and so move more rapidly
Which molecule has higher bpt question - key points
- If similar size = similar dispersion forces
- Most polar bonds
- Strongest IM interactions (what is it)
Therefore, highest bpt
Which molecule moves fastest on chromatography column - key points
- Least polar molecule
- No polar bonds
- Interacts least with polar stationary phase
Therefore moves fastest on chromatography column
Types of curly arrows
Half arrow = 1 electron moving
Full arrow = 2 electrons moving
Homolytic bond cleavage
One electron from the bond ends up on each of the atoms which were formally bonded (i.e. covalent bond cleaves symmetrically)
Heterolytic bond cleavage
Both electrons from the bond end up on one of the atoms which were formally bonded (i.e. covalent bond cleaves asymmetrically)
If ‘A’ is more electronegative than C, what happens?
CH3C-A
Bonds will break:
CH3C+ + A- (carbocation + anion)
e.g. A = Cl, O, Br
If ‘A’ is less electronegative than C, what happens?
CH3C-A
Bonds will break:
CH3C- + A+ (carbanion + cation)
e.g. A = metal
Geometry of carbocations
Planar (sp2 hybridised)
Relative stability of carbocations
Tertiary > Secondary > Primary > Methyl
Classes of reaction + sub-classifications
Substitution
Addition
Elimination
Polar/non-polar (radical)
Involving a nucleophilic/electrophilic reagent
Nature of substrate (alkyl, acyl, aryl)
Substitution reactions
Where one atom/group is replaced by another through breaking and formation of sigma bonds
Addition reactions
A pi-bond breaks, and is replaced by 2 sigma bonds
Elimination reactions
Two sigma-bonds break, and one pi-bond forms
Reaction mechanism
The detailed pathway by which reactants are converted to products
Often requires multiple steps (elementary reaction) that occur sequentially
Reactive intermediates
Species involved in a reaction mechanism that may be unstable or short-lived
Nucleophilic substitution
Reagent is a nucleophile
SN1 occurs step-wise (bond-breaking then bond-forming)
SN2 occurs in a single step (bond-breaking/forming simultaneous)
Nucleophile
Electron rich –> electron donor
Can be neutral or negatively charged
Have either lone pairs of e- or e- in pi-bonds
Electrophile
Electron deficient –> electron acceptor
Can be neutral or positively charged
Polar reaction occurs when…
A nucleophile reacts with an electrophile to form a new covalent bond
The terms cis and trans are used to describe…
The relative position of substituents on a ring
