Test 2 Flashcards
Describe the two different types of isomers
Constitutional isomers: isomers with different atomic connectivity
Stereoisomers: isomers with the same atomic connectivity but with differed 3D arrangement
define an isomer
different compounds with the same molecular formula
define chirality
the ability of objects to exist as non-superposable mirror images of each other.
- an object that contains a mirror plane and can be superimposed with its mirror image is achiral
- an objects that does not have a mirror plane and cannot be superimposed with its mirror image
define enantiomers
chiral molecules and their non-superimposable mirror images
what does chirality often originate from?
an atom that is connected to four different substituents - called a stereogenic centre, chirality centre, or stereo centre
do enantiomers have identical physical and chemical properties?
yes; melting points, boiling points, and density are identical. The IR and NMR spectra of enantiomers are also identical
how can enantiomers be differentiated from each other?
- interactions with other chiral molecules
- optical activity (rotation of plane-polarised light by same angle but in opposite directions, + or -)
optical rotation α
the angle that a sample rotates plane-polarised light
- clockwise rotation (α > 0): dextrorotary (+ or d)
- counterclockwise rotation (α < 0) levorotary (- or l)
define a racemic mixture
a 1:1 mixture of two enantiomers
does a racemic mixture rotate polarised light?
no; the single enantiomers rotate light in opposite directions and cancel each other out
how do you determine the absolute configuration of a chirality centre?
- identify the chirality centre
- determine the priorities of attached groups
- rotate the molecule to put the lowest priority group (number 4) at the back
- determine the direction of priority group numbers 1,2, and 3
if clockwise -> R (right)
if counter-clockwise -> S (left)
define diastereomers
stereoisomers that are non-superimposable, non-mirror images
how do diastereomers differ from enantiomers?
enantiomers: changing configuration at all stereocenters
diastereomers: changing configuration at some stereocenters
do diastereomers differ in their physical and chemical properties?
yes; boiling point, melting point, IR and NMR spectra, solubility, etc
maximum number of stereoisomers =
2^n, where n = number of chirality centres
define a meso compound
- a molecule that has chirality centres but is a chiral due to an internal mirror plane
- the molecule and its mirror image are superimposable = achiral
double bond isomerism
E = highest priority groups are trans
Z = highest priority groups are cis
chirality depends on
whether the molecule lacks a plane of symmetry and has a non-superimposable mirror image.
do diastereomers have optical activity?
yes - diastereomers have different physical properties including optical activity
Define a nucleophile
an electron rich species (with a lone pair or pi bond) that reacts by donating an electron pair to an electron-poor species
Define an electrophile
an electron poor species (polarised bond or empty orbital) that reacts by accepting an electron pair from a nucleophile
Is a carbonyl carbon a nucleophile or an electrophile?
Overall, the molecule is an electrophile.
Electron pairs always move from
a nucleophile (high electron density) to an electrophile (low electron density)
when a bond is broken, bonding electrons tend to move toward which atom
the more electronegative atom
define polarisability
the ability to shift bonding or nonbonding electrons in response to nearly nucleophile or electrophile
as you go down a group
size increases and polarisability increases (more reactive bonds)
as you go across a period
size decreases and electronegativity increases (stabilizes negative charge better)
why do reactions often involve polar bonds?
due to polarisability and differences in electronegativity
intermolecular reactions
reactions that occur between two or more molecules
intramolecular reactions
reactions that occur between two functional groups on the same molecule
state the equation for the equilibrium constant
K(eq) < 1
reactants are more favoured
K(eq) > 1
products are more favoured
how is the equilibrium constant K related to the Gibbs free energy change?
ΔG = -RTlnK(eq)
K(eq)>1 and ΔG<0: products are favoured (reaction is exergonic)
K(eq)<1 and ΔG>0: reactants are favoured (reaction is endergonic)
exergonic
- reactions where there is a net release of free energy
- spontaneous
- ΔG<0
exothermic
- reactions where there is a net release of heat
- ΔH<0
when ΔH is negative:
- exothermic (heat released)
- bonds formed in product are stronger (more stable) than bonds broken in reactants
when ΔH is positive:
- endothermic (heat absorbed)
- bonds formed in products are weaker (less stable) than bonds broken in reactants
bond dissociation energy
the amount of energy required to symmetrically break a covalent bond
define entropy (ΔS)
measure of freedom of movement or disorder
what is the effect of ΔS being positive on ΔG?
there is more movement/disorder and ΔG becomes more negative
ΔS<0
entropically unfavourable
ΔS>0
entropically favourable
transition state
- highest energy structure
- in-between reactants and products
- from transition state, reaction can go in either direction
- cannot be isolated or observed
ΔG‡
- activation energy
- energy required to reach transition state
- determines the rate of reactions (higher activation E = slower process)
what is the rate-determining step of the reaction?
the slowest elementary step
what is the effect of a catalyst?
increase reaction rate without changing ΔG of the overall reaction
- catalyst not consumed during the reaction
- activation energy lowered by providing a new reaction mechanism
define a Bronsted acid and a Bronsted base
Bronsted acid: proton (H+) donor
Bronsted base: proton (H+) acceptor
how can the position of an acid-base equilibrium be determined?
by comparing the acid strengths. strong acids dissociate more readily than weak acids, so the equilibrium will lie in the direction of the weaker acid and base
draw two free energy graphs for strong and weak acids
what is the ΔG for strong/weak acids?
strong acids have a negative ΔG
weak acids have a positive ΔG
how does pKa relate to acid strength?
the lower the pKa value, the stronger the acid
how does stability of a conjugate base relate to the strength of an acid?
conjugate bases that are stabilised have lower free energy than similar bases without the stabilising effect (->strong acid)
how does electronegativity impact acid strength?
conjugate bases in which the atom carrying the negative charge is more electronegative are more stable (weaker base). this leads to a stronger acid.
high electronegativity -> increase ability to accomodate negative charge
how does induction impact acid strength?
removal of electron density from an atom by a strongly electronegative atom nearby increases the ability to accommodate negative charge and increases stability. this leads to a stronger acid.
how does hybridisation impact acid strength?
- orbitals with higher ‘s’ character are lower in energy because s orbitals experience a greater effective nuclear charge
- conjugate bases with unpaired electrons in orbitals with greater ‘s character’ are more stable
- this leads to a stronger acids
s character
sp = 50%
sp2 = 33%
sp3 = 25%
how does resonance impact acid strength?
- charge delocalisation increases the ability to accommodate negative charge and increases stability for the conjugate base
- this leads to a stronger acid
common acids and their pKa values
strongest acid
HCl (-7)
H3O+ (0)
carboxylic acid (5)
phenol (10)
water (16-18)
CH4 (>45)
weakest
define a Lewis acid and base
Lewis acid: electron pair acceptor
Lewis base: electron pair donor
examples of Lewis acids
- tricoordinate B and Al
- cations such as Li+, Mg2+
examples of Lewis bases
- lone-pair donors
- benzene
