Revision Hybridisation -The valence electrons of a Carbon atom sit in 1s 2 2s 2 2p 2 orbitals that are different in energy. It has 2 x 2s electrons + 2 x 2p electrons are available to form 4 covalent bonds. These orbitals mix (hybridise) to form new orbitals. 1 sp3 hybridisation Alkanes If the 2s orbital mixes with three 2p orbitals 1 x s + 3 x p = 4 x sp3 = 4 identical orbitals = 4 σ orbitals 4 σ bonds = 4 single bonds 2 sp2 hybridisation Alkenes If the 2s orbital mixes with two 2p orbitals 1 x s + 2 x p = 3 x sp2 (+ 1 x p unhybridised) = 3 identical orbitals + 1 different orbital = 3 σ orbitals + 1 π orbital 1 σ bond + 1 π bond = 1 double bond
3 sp hybridisation Alkynes, Allene If the 2s orbital mixes with one 2p orbital 1 x s + 1 x p = 2 x sp (+ 2 x p unhybridised) = 2 x identical orbitals + 2 different orbitals = 2 σ orbitals + 2 π orbitals 1 σ bond + 2 π bonds = 1 triple bond -π bond is weaker (more unstable higher energy form) than σ bond as the σ orbitals overlap better than the π orbitals which can only overlap from the sides. Spectroscopy Spectroscopy Mass spectrometry IR spectroscopy UV-Vis spectroscopy NMR Identification and information mass of molecule and its fragments = molecular mass = structural formula (reveals Br) IR absorption wavelength = type of bond = functional groups If absorbed = conjugation Exact chemical environment: atom-by-atom connectivity =how many C environments, how many Cs in each of that environment, how many and to what the C is bonded to = how many H environments, how many Hs in each of that environment, how many neighbours each H has, what each H is bonded to Acid-Base -Bronsted-Lowry model: Acid = Proton donor -Lewis model: Acid = Electron pair acceptor Base = Proton acceptor Base = Electron pair donor -The more equilibrium lies to the right = More [H 3 O + ] = Higher K a = Lower pk a = Stronger acid
Polarity influences reactivity. F, O, Cl and N are most electronegative form polar bonds to carbon (and hydrogen) -Inductive effect = Transmission of charge through bonds F, O, Cl and N are all more electronegative than C and all inductively pull electrons from the carbonyl C. Molecules with these tend to be more reactive/ stronger acid. Stereochemistry Isomer Constitutional Isomer Different sequence of bonding/ nature -Number of constitutional isomers increase rapidly with number of C -Physical and chemical properties differ particularly when different functional groups are present Stereoisomer Different arrangement of same groups in space -same nature and sequence of bonding Conformational Isomer Differ by rotation about a single bond and can be interconverted Configurational Isomer Interconverted only by breaking and remaking bonds Enantiomers Non-superimposable mirror images -Identify the stereogenic centre -Assign priority to each group directly attached to the centre in order of atomic number -If identical, work along the chain -Multiple bonds are expanded to the same number of single bonds Diastereoisomer Not mirror images + superimposable Assign priority to each group on each carbon of C=C -The higher the atomic number of the atom attached, the higher the priority -If identical atoms attached, work along the chain until the first point of difference, then go by atomic number R (clockwise) S (anti-clockwise) Z (cis) E (trans) -Orientate the molecule with lowest priority group at the back and trace 1,2,3 -Orientate the molecule with lowest priority group at the back and trace 1,2,3 If high priority groups are on the same side of C=C If high priority groups are on the opposite sides of C=C Resonance = Delocalisation of electrons across several atoms ( ) -Individual resonance forms are representations that satisfy valency rules. The real structure is a hybrid/ average of all resonance structures. -Resonance forms vary only by the position of their π and non-bonding electrons: all atoms remain in the same place and no sigma bonds are broken.
-Resonance imparts stability - The π electrons are shared/ charge is evenly spread across the whole system to allow lower energy form. -If conjugate base is resonance stabilised, the acid is much stronger. -Curly arrows relate the location of electrons in the different resonance forms -Only electrons move (in lightning fast speed) Tautomers -Tautomers = isomers that interconvert rapidly by the migration of hydrogen from one atom to another -Isomers are in equilibrium (equilibrium arrows NOT resonance arrows) -An atom moves Use of curly arrows = a pair of two electrons in motion -A curly arrow starts at the electron pair that moves and ends at the atom to which the electron pair has moved -Arrow from a bond = bond breaks -Arrow between two species = new bond formed between them -Charge must be equal on both sides Nucleophiles -Nucleus loving = chase positive charge (+ or δ+) -Negatively charged ( or δ ) = electron/ electron pair rich = supplies an electron pair Electrophiles -Electron loving = chase negative charge ( or δ ) = seeks an electron pair -Positively charged (+ or δ+)
1. Organic Reactions and Reactivity (13L) Organic reactions Chemical reactions are all about electrons. A reaction is an electron auction. A mechanism shows the exact sequence of steps in a reaction (represented by curly arrows). Reaction mechanisms: 1) Polar -Important polar bonds: -Electrophiles: hydrogen/ proton H +, C in alkyl halides, aldehydes, ketones and carboxylic acid derivatives -Nucleophiles: the π bond in alkenes, lone pairs on alcohols, amines and hydroxide ion Nucleophilic substitution reactions: S N 1 and S N 2 reactions Many factors can control reactivity: 1) Substrate 2) Nucleophile or base: -For ONE specific atom, nucleophilicity follows order of basicity
-Negatively charged species are more nucleophilic -Nucleophilicity increases down the periodic table: with increasing size and decreasing electronegativity, the outer shell electrons are less tightly held and are more polarizable (softer) 3) Leaving group -More stable anions, better leaving group -Lower C-LG bond strength and greater polarizability, better leaving group -The factors that stabilize anions can also stabilize partial charges 4) Solvent -Protic solvents are capable of donating H s (H 2 O, CH 3 OH) -Aprotic solvents are not capable of donating H s (CH 3 C N, (CH 3 ) 2 NCHO, (CH 3 ) 2 S=O, DMK) 1) First order: S N 1 mechanism -Involves 2-steps substitution of an alkyl halide by nucleophile via carbocation intermediate.
1. The C-X bond breaks and the halogen leaves 2. Then the nucleophile arrives and its bond to C forms -Spontaneous dissociation of the alkyl halide occurs in a slow, rate-limiting step to generate a carbocation intermediate in a planar arrangement. The carbocation reacts with the nucleophile in a fast step. -Two transition states = highest energy points (local maxima) -It proceeds with racemisation If the product is chiral, it will be racemic. (As this mechanism involves an intermediate, the nucleophile arrives and has 2 choices of bonding location at an equal probability. This forms a 50:50 racemic mixture.) -Reaction rate is determined by the concentration of the electrophile only -Unimolecular: Rate = k x [substrate] a) Substrate -Reaction rate decreases in the order of 3 >> 2 > 1 methyl for the alkyl halide -S N 1 is favoured for tertiary alkyl halides. The intermediate in S N 1 reactions is a carbocation. Hyperconjugation stabilises substituted carbocation (the electron density in the adjacent C-H bonds overlaps with the empty p orbital of the cationic C) i.e. carbocations are the most stable with 3 alkyl groups. Hence, the halogen leaves. -Reaction rate increases with carbocation stability. Factors that stabilise carbocations will also stabilise the developing carbocationic character in transition state 1. This will lower its energy and therefore E a and increase the rate.
b) Nucleophile The nucleophile concentration is not in the rate equation; it is not involved in the rate determining step and hence does not influence the rate of reaction. c) Leaving group -Reactions can occur under acidic conditions and this can convert a poor leaving group into a good leaving group. -More stable anions are better leaving groups (Lower energy LG lowers the activation energy to intermediate and increases rate) d) Solvent -Charge separation increases in S N 1 transition state 1 -Polar solvents stabilise carbocation intermediate by orienting around the positively charged carbocation. 2) Second order: S N 2 mechanism -Involves single step substitution of alkyl halide by nucleophile via high energy transition state (no intermediate) -Transi on state (labelled ) is the highest energy point on the lowest energy pathway from star ng materials to products
1. The nucleophile arrives (its bond to C forms) and the halogen leaves (its bond to C breaks) at the SAME TIME -The nucleophile uses its lone pair electrons to attack the alkyl halide carbon 180 from the departing halogen. This leads to a transition state with a partially formed C-nucleophile bond and a partially broken C-LG bond and in planar arrangement before returning to its tetrahedron. -It proceeds with inversion If the product is chiral, its absolute configuration will be inverted. (As two reactions happen simultaneously, the halogen will leave in one side of the alkyl halide whilst the nucleophile arrives on the other side, 180 to the C-LG bond. It will invert the molecule from S to R or R to S.) -Reaction rate is determined by concentration of both nucleophile and substrate -Bimolecular: Rate = k x [nucleophile] x [substrate] a) Substrate -S N 2 is favoured for primary alkyl halides -Reaction rate decreases in the order methyl 1 > 2 >> 3 for the alkyl halide substituents block the approach of the incoming the incoming nucleophile -Sterically, the nucleophile cannot react with tertiary alkyl halide. Primary alkyl halide provides space. -The HOMO electron orbital of the nucleophile interacts with the LUMO electron orbital (antibonding sigma bond σ*) of the alkyl halide. b) Nucleophile -The rate equation involves the concentration of nucleophile -Higher energy nucleophile lowers the activation energy and increases rate -anions, basic, increasing size c) Leaving group -More stable anions are better leaving groups -Lower energy LG lowers the activation energy and increases rate d) Solvent -In the transition state, the negative charge becomes shared over the nucleophile and leaving group. This contrasts with the starting materials where charge is centred on the nucleophile. -Solvents of high polarity would tend to stabilise the nucleophile more than the transition state and so slow the reaction rate. Solvents of low polarity would increase the reaction rate. -Aprotic is favoured over protic -Protic solvents contain OH or NH bonds. Solvation involves hydrogen bonding and stabilisation of the nucleophile. This lowers the energy of the starting materials, increases E a and slows the rate.
-Aprotic solvents (no OH or NH bonds) solvate cations better than anions. This decreases ion pairing of nucleophile leading to naked anions. Anions are better nucleophiles. This increases the energy of the starting materials, decreases E a and increases the rate. It is possible for S N 1 and S N 2 mechanisms to compete under certain conditions. Hydrolysis of the following series of compounds is observed to occur by the following mechanisms: In 50% acetone/h 2 O the benzyl chloride reacts with competing SN1 and SN2 mechanisms. In H2O, a more polar protic solvent, benzyl chloride reacts by and SN1 mechanism. Furthermore, it is possible that reactions proceed with mixed SN1 and SN2 character. That is, a substitution can occur by an S N 2 mechanism where bond breaking occurs in advance of bond breaking - the reaction can be described as having S N 1 character. Energy diagram for nucleophilic substitution -Transition state: highest energy species; cannot be isolated -Intermediate: local energy minimum (though still high energy species); Can sometimes be isolated -Activation energy: difference in energy between starting materials and the transition state -Rate determining step: in a multistep reaction, the step in a reaction with highest activation energy (barrier)
- G is the ac va on energy. -If G is small, then reac on is fast. If G is large then reaction is slow. -The size of G depends highly on the energy of the transi on state. -NEVER change the energy of the product Hammond Postulate The structure of a transi on state (TS ) resembles the structure of the nearest stable species (in energy). -The transition state for an endothermic reaction step will resemble the products (late transition state) -The transition state for an exothermic reaction step will resemble the starting materials (early transition state) 1) S N 1 -Both TS 1 and 2 are close in energy to the carbocation intermediate. -The transition states therefore resemble a carbocation in structure. -Therefore factors that stabilise carbocations will also stabilise the transition states. -Formation of the carbocation is endothermic. TS 1 is late and resembles the carbocation. -The rate determining step involves TS 1. Hence A more stable carbocation intermediate Lower energy TS 1 Lower G Faster reaction -Reaction of the carbocation with a nucleophile is exothermic. TS 2 is early and resembles the carbocation. -A more stable carbocation intermediate Lower energy TS 2 -When TS 1 drops in energy, TS 2 follows Rate determining step stays the same 2) S N 2
-TS is early and resembles the starting materials. -Higher energy substrate Lower G Faster reac on Application of the Hammond postulate Radical halogenation reflects the Hammond postulate: -Radical reactions occur via chain mechanisms that involve initiation, propagation and termination. -Termination destroys radicals and stops the propagation reaction. To minimise termination steps, radical reactions are conducted with very low concentration of radicals the chance of radicals meeting radicals is low.
-For Br, the reaction is endothermic and hence the transition state resembles the product. To stabilise the transition state (and to increase the reaction rate), the more stable product is preferred; hence the 2-alkyl radical is the major product. -For Cl, the reaction is exothermic and hence the transition state resembles the starting material. The stability of the end product is inconsequential to the stability of the transition state hence the reaction is non-selective. Kinetic vs Thermodynamic reactions -Kinetic product has smaller G (height of energy barrier) -Thermodynamic product has higher stability of the product (lower energy product) 1) Moderately exothermic reaction -The activation energies for forward and reverse reactions are the similar Low temperatures and short reaction times (products form irreversibly): -Product Z is the major product and a kinetic product as it has a lower G than Y High temperatures and long reaction times (products at equilibrium): Product Z is the major product and a thermodynamic Product Y is the major product and a thermodynamic product as it has lower energy and thus more stable product as it has lower energy and thus more stable
2) Strongly exothermic reaction -The activation energy for forward reaction is much smaller than the activation energy for reverse reaction -Products form irreversibly, following the rules of kinetics only. -Product Z is the major product and a kinetic product as it has a lower G than Y Conformational analysis Differences in energy -Energy is the highest during the eclipsed conformation due to steric interaction that comes from repulsion between electron densities. -Energy is the lowest during the staggered conformation when the substituents are furthest away from each other. -The torsional strain is caused by hyperconjugation. The electron density in the filled C-H σ bond fill the empty C-H σ* antibonding orbital. If a molecule with a stereocentre (with H attached) is spun about the sp 3 bond, the H will experience all different environments. The H s undergo coupling and show up in NMR spectrum differently. They are called diastereotopic hydrogens. Cyclic saturated alkanes: C n H 2n -Rings have very limited conformational freedom compared to their acyclic counterparts.