CHM185 Chemistry for Engineering Students RevisionOrganic ChemistryDr J Wilkie:STRUCTURE AND BONDING (tick when completed)-Simple representation of structure, line formulae - Atomic and molecular orbitals shape and phase - Covalent bonds - Hybridisation, , bonding and antibonding orbitals STEREOCHEMISTRY- Isomers constitutional isomers, E/Z isomerism, stereoisomers, enantiomers, diastereoisomers. - Dotted line/wedge notation - Chirality - R and S descriptors - Conformaion, staggered and eclipsed, axial and equatorial - Newman projections REACTIONS AND MECHANISMS- Curly Arrow/ Arrow pushing representations - Electrophiles, nucleophiles and radicals - Polarisation and electron distribution - Substitution reactions, SN1 and SN2 - Eliminations E1 and E2 - Carbon metal bond formation (Grignard Reagents) - Reactions with carbonyl compounds ALKENES- Geometrical Isomerism, cis and trans, Z/E-Descriptors - Additions reactions (Hydration , reactions with halogens and hydrogen halides , Hydroboration , Hydrogenation and Epoxidation - Dienes: Addition reactions and Diels Alder reactions ALKYNES- Addition reactions - Acidity

STRUCTURE AND BONDINGHydrogen Atomic OrbitalsEach orbital contains a maximum of two electrons. These two electrons must have opposite values for the spin, generally indicated by an up arrow or a down arrow. When filled, the first shell (one 1s orbital) holds two electrons, the second shell (one 2s and three 2p orbitals) holds eight electrons and the thirds shell (one 3s, three 3p orbitals, and five 3d orbitals) holds eighteen electrons.

- The Aufbau principle (aufbau means building up in german, extra points) explains the order in which electrons fill the various orbitals in an atom. Filling begins with the orbitals in the LOWEST ENERGY, or MOST STABLE shells, and continues through the higher-energy shells, until the appropriate number of shells is filled for each atom. Thus, the 1s orbitals fill first, then the 2s, followed by the 2p and then the 3s.

Hunds rule says that each degenerate orbital 2px, 2py and 2pz, must FIRST RECEIVE ONE ELECTRON BEFORE ANY OF THE ORBITALS CAN RECEIVE A SECOND ELECTRON

- Orbitals are described by quantum numbers these must be unique for each orbital An atomic orbital is defined by a unique set of three quantum numbers (n, l and ml). A fourth quantum number ms gives information about the spin of an electron in an orbital and specifies how many electrons can occupy that orbital.>> Principal Quantum Number (n): n = 1,2,3

- This specifies the energy of an electron and the SIZE of the orbital. The principal quantum number may have any positive integral value between 1 and infinity. All orbitals that have the same value of n are said to be in the same SHELL (level).

>> The Orbital, or Angular Momentum Quantum number (l)- The quantum number l is called the orbital quantum number. For a given value of the principal quantum number, the allowed values of l are positive integers lying between 0 and (n-1). Therefore is n=3, the permitted values of l are 0,1 and 2. Each value of l corresponds to a particular type of atomic orbital. The value of l provides information about the shape of the orbital and the angular momentum of the electron within the orbitaln=1 l=0n=2 l= 0,1n=3 l = 0,1,2n=4 l = 0,1,2,3,>> The Magnetic Quantum Number (ml): ml = -l,.,0,.,+l- The magnetic quantum number m1 relates to the directionality of an orbital and has values which are integers between +l and l. If l=1, the allowed values m1 are -1, 0 and +1.>> The Electron Spin Quantum Number (ms): ms = + or - This specifies the orientation of the spin axis of an electron, up or down arrows. No two electrons can have a set of identical quantum numbers. Each orbital can accommodate two electrons, one of each and opposite spins.Deriving quantum numbers and shit.1) Derive possible sets of quantum numbers for n=2, and explain the significance of these sets of numbers- Let n=2i) Remember that a value of n defines an energy or principal level.ii) Possible values of l lie in the range of 0 to (n-1). Therefore for n=2, l = 0 and 1iii) This means that the n=2 level gives rise to two sub levels, one with l=0 and one with l=1iv) Now determine the possible values of the quantum number ml. We know values of ml lie in the range l0+lThe sub level l=0 has associated with it ONE value of mlml = 0 for l=0The sub level l=1 has associated with it THREE values of mlml = -1, 0 or +1 for l=1The possible sets of quantum numbers for n=2 areNLml

200

21-1

210

21+1

Total of 4 orbitals in second energy level, it can accommodate 8 electrons. 2 in the 2s orbital and 6 in the 2p orbitals- The physical meaning of these sets is that for the level n=2, there will be two types or orbital (because there are two values of l). For l=1, there will be three orbitals all of similar type but with different directionalities.

COMBINING ORBIALS TO MAKE BONDS Orbitals can overlap in phase or out of phase. During bonding, wave functions with the same sign overlap in an IN-PHASE OVERLAP, and wave functions with opposite signs overlap in an OUT-OF-PHASE OVERLAPIn-phase overlap is a constructive, or bonding overlap of atomic orbitals. In this overlap, the wave functions reinforce one another. This reinforcement increases the probability of finding the electrons in the regions between the two nuclei. The molecular orbital that results from an in-phase overlap is a BONDING MOLECULAR ORBITAL. This is a representation of the 1s orbitals of two hydrogen atoms forming a bonding molecular orbital. In a bonding molecular orbital two or more in-phase orbitals overlap to form a bond An out-of-phase overlap forms and ANTIBONDING MOLECULAR ORBITAL. With an out-of phase overlap, a node develops between the two nuclei. For each bonding molecular orbital that forms, an antibonding molecular orbital also forms. This is an OUT-OF-PHASE overlap of the 1s orbitals of two hydrogen atoms forming an ANTIBONDING MOLECULAR ORBITAL.- Usually, an antibonding molecular orbital contains no electrons because it DESTABILISES the bond. Generally, molecules in the lowest energy state have empty antibonding orbitals.- To illustrate this concept, we will look at the bond between two hydrogen atoms in a H2 molecule. The 1s ATOMIC orbital of EACH HYDROGEN combines and generates the two hydrogen-hydrogen MOLECULAR ORBITALS one bonding and one antibonding. The two molecular orbitals of a H2 molecule are generated by combining two 1s atomic orbitals. One of the molecular orbitals is BONDING and is LOWER IN ENERGY. The other is antibonding and is higher in energy. The arrows represent the electrons involved in forming the bonding molecular orbital. Electrons prefer to occupy the orbital with the LOWEST POSSIBLE energy state, both electrons reside in the bonding molecular orbital of a hydrogen molecule.

HYBRIDISATION OF ATOMIC ORBITALS HYBRID ORBITALS are spatially directed orbitals which may be used to produce LOCALISED BONDS in a valence bond scheme, they are the individual orbitals formed from hybridisation. The three types of orbital hybridisation are sp, sp2, sp3.i) sp in sp hybridisation TWO orbitals are involved, one s and one pii) sp2 in sp2 hybridisation THREE orbitals are involved, one s and two p orbitals

iii) sp3 in sp3 hybridisation FOUR orbitals are involved, one s and three p orbitals- Because hybridisation blends all the characteristics of the s and p orbitals, the name of the new orbital indicates what PROPORTION of each orbital is like an s orbital and what portion is like a p orbital. With sp2 hybridisation each hybrid orbital bears 33.3% of the s orbitals characteristics and 66.6% of the p orbitals characteristics. Another consideration with hybridisation is the shape of the hybridised orbitals. The FOUR hybrid sp3 orbitals have a shape that is a combination of the s and p orbital shapes, as illustrated below

- Similar to p orbitals, each sp3 orbital (there are four of them) has two lobes, but unlike the p orbitals they are of UNEQUAL size. This means that for each orbital there is a greater electron density on one side of the nucleus than on the other. This unsymmetrical electron density allows for greater overlap and the formation of STRONGER BONDS than is possible with an unhybridised orbital.

PI AND SIGMA BONDS- Using the illustrations above let us consider the hybridisation in ethane. Ethene requires 3 hybrid orbitals (3 C-H bonds) so we combine 3 atomic orbitals (1 x s, 2 x p) so sp2 hybridisation.What happens to the sp2-orbitals on the C atom? They overlap END-ON forming a SIGMA BONDAnd the p orbitals? They overlap SIDE-ON giving a PI BOND

A Double Bond consists of one SIGMA BOND and one PI BONDEssentiallyC bonded to 4tings(NOT C) = sp3C bonded to 3tings(NOT C) = sp2C bonded to 2tings(NOT C) = sp

LONE PAIRS- Let us look at the oxygen atom in a carbonyl groupCarbon has hybridisation of sp2Oxygen has:1 x sigma bond hybrid1 x pi bond p orbital2 lone pairs hybrids.Total of 3 hybrid orbitals on the oxygen, must be sp2

What about the oxygen in furan?- It also has two lone pairsIn furan the oxygen has:2 x sigma bonds 2 hybrids0 x pi bonds 02 x lone pairs 1 hybrid and 1 p-orbital

3 hybrids therefore must be a sp2 oxygen

Why are the different?We can draw ALTERNATIVE BONDING PATTERNS that involve one of the lone pairs in a pi bond

HYBRIDISATION RULES1) sigma bonds are formed from HYBRID orbitals2) pi bonds are formed from p-orbitals3) lone pairs that can form pi bonds in alternative structures are in p and hybrid orbitals4) lone pairs that cannot form pi bonds in any alternative structure are in hybrid orbitalsRESONANCE- Alternative ways of drawing molecules that still satisfy the octet rule. The real structure is an average of all the possible resonance structures. Some structures may contribute more than others if they are MORE STABLERULES FOR RESONANCE STRUCTURES-Only pi bonds and LONE PAIRS can move-You CANNOT break sigma bonds- The connectivity must not change. SummaryAtomic Orbitals- Determined for the hydrogen atom-Level 1 has only 1 s-orbital-Level 2 has 1 x s and 3 x p orbitalsHybridisation- Same number of hybrids out as atomic orbitals in- Always use an s-orbital and however many o-orbitals required- sigma bonds ALWAYS hybrid, pi ALWAYS p-orbitals, lone pairs either

Hybridisation and shape- sp3 is TETRAHEDRAL-sp2 is TRIGONAL PLANAR- sp is LINEAR

ALKYLHALIDES: SUBSTITUTION AND ELIMINATION REACTIONSAlkylhalides:- What are they?- How are they formed?- Chemical reactions- Reactivity of different alkylhalidesReactions of alkylhalides

- SN2substitutions-SN1 substitutions - Factors that decide which substitution mechanism- E2 elimination - Factors that decide between elimination and substitution- E1 elimination - Factors that decide between elimination mechanisms

The general formula for an alkylhalide, otherwise known as a halogenoalkane, is RX where R is an alkyl group and X is F, Cl, Br or I; the halogens.Halogens have seven valence electrons. The halogens have very high electronegativities and are very reactive, especially with the alkali metals.FORMATION OF ALKYLHALIDESRadical Halogenation. - Simple nucleophillic substitution reaction involving the halogen anion acting as a nucleophile

Addition to alkenes both symmetrical and unsymmetrical

Reactions of alkyhalides- The halide ion is a good leaving group so it is easily substituted by nucleophiles.

Direct substitution in a single step: SN2 mechanism- In an SN2 reaction, a new bond is formed between the nucleophile and the carbon atom, while the carbon-halogen bond is broken. We call the chlorine the leaving group. The species being attacked by the nucleophile is the electrophile. - The term SN2 stands for Substitution Nucleophillic 2nd Order (also called bimolecular). According to the SN2 mechanism, there is a single TRANSITION STATE because bond-breaking and bond making occur SIMULTANEOUSLY for this to occur, the nucleophile must approach from the backside of the carbon-leaving group bond:

- Note that there is NO INTERMEDIATE in an SN2 reaction, just a TRANSITION STATE. A transition state has no lifespan and is just the highest ENERGY POINT on the reaction profile as starting materials transition into productsRate Equation for SN2 reactions:rate = k[R-X] [Nuc] As you can see from the rate equation, rate depends on the CONCENTRATION of both the nucleophile and the akylhalide. It is a BIMOLECULAR REACTION, one which two reactant take part in the transition state of the SLOW RATE-DETERMINING STEP of the reaction. Note: [ ] means concentration, remember?

SUBSTITUTION IN TWO STEPS: SN1 MECHANISM SN1 indicates a substitution, nucleophilic, unimolecular reaction, described by the expression Rate = k [R Leaving Group]. This implies that the rate determining step of the mechanism depends on the decomposition of a single molecular species SN1 is a multi-step process with the following characteristics: The carbon-halogen bond is broken first, and there is a SLOW loss of the leaving group forming a HIGH ENERGY carbocation intermediate. Then there is a rapid of a nucleophile on the electrophilic carbocation to form a new bond.

STEREOCHEMISTRY OF SN1 REACTIONS (do you know what stereochemistry means yet?) In an SN1 reaction, the nucleophile attacks the PLANAR carbocation. Since there is an EQUAL PROBABILITY of attack on each face of the plane there will be a LOSS OF STEREOCHEMISTRY at the reactive centre as both products will be observed.

Because the mechanism goes through a carbocation, the leaving group must be attached to either a tertiary or secondary carbon to stabilise the intermediate. A methyl or primary leaving group will NOT form a carbocation

DECIDING BETWEEN SN2 AND SN1- SN1 mechanism involves formation of a carbocation intermediate- Stabalising the intermediate favours an Sn1 mechanismWHY ARE TERTIARY CARBOCATIONS MORE STABLE? - Alkyl groups are ELECTRON DONATING by induction. They push electrons away from themselves. This means the alkyl group becomes slightly positive and the carbon they are attached to becomes slightly negative. The alkyl group has a positive inductive effect:

- This spreads the positive charge over the entire molecule, rather than keeping it on one atom. If the carbocation is primary there will only be one electron-donating alkyl group. If it is secondary, there will be two, tertiary there will be three. The more electron donating groups present, the more stable the ion will be.

LEAVING GROUPS

Halide XC-X Bond strengthpKa of HX

F118+3

Cl81-7

Br67-9

I54-10

- Reactions are easier if we are breaking a C-I bondOH is a very bad leaving group, how can we make it leave? We can ask it politely, or simply change the CONDITIONS. We can protonate the alcohol to make the leaving group water, so that we can then substitute the OH for our choice of halogen

ELIMINATION REACTIONSE2 REACTIONS- The term E2 stands for elimination bimolecular. E2 eliminations, in contrast to E1 mechanisms are PROMOTED BY A STRONG BASE. The base is VITAL to the reaction; it is directly involved in the RATE-DETERMINING step. The reaction is BIMOLECULAR because TWO MOLECULES must come together for the reaction to occur. The mechanism of an E2 elimination is shown below:

E2 mecanism:- Remember the base attacks the NEIGHBOROUGHING C-H bond and removes the H at the SAME TIME the alkene double bond starts to form and the SAME TIME the X group starts to leave. THREE ARROWS.Rate = k[R-X][Base]Stereochemistry Of Elimination reactions- Elimination is antiperiplanar. It is the staggered arrangement, aligning two sigma bond that become the pi bond. E1 REACTION MECHANISM- E1 indicates an elimination unimolecular reaction, where rate = k[R-X]. This implies that the rate determining step depends on the decomposition of a single molecular species. Overall it is a multistep process with two critical steps:1) The loss of a leaving group LG to generate a carbocation intermediate2) The loss of a proton, H+ from the carbocation to form the pi bond

How Various Components Affect The Reaction Pathway1) Reactivity order: 3 > 2 > 1- In an e1 reaction the rate determining step is the loss of the leaving group to form the intermediate carbocation. The more stable the carbocation is, the easier it is to form, and the faster the e1 reaction will be2) Leaving group- The only event in the rate determining step of the e1 reaction is the breaking of the C-LG bond. Therefore, there is a very strong dependence on the nature of the leaving group. In the acid catalysed reactions of alcohols, the OH is protonated to give an OXONIUM ION (any oxygen cation with three bonds), providing the much better leaving group of water3) BaseSince the base is NOT involved in the rate determining step, the nature of the base is unimportant in an E1 reaction. However, the more reactive the base, THE MORE LIKELY AN E2 REACTION BECOMES. E1 or E2?- E1 reactions usually favour the more STABLE alkene as the MAJOR product. E1 favours producing E over Z isomers.The E1 pathway is most common with:- GOOD LEAVING GROUPS- MORE STABLE CARBOCATIONS- VERY WEAK BASES

CHOOSING BETWEEM E2, SN2, AND E1/SN1- How do we choose between elimination and substitution mechanisms and how can we predict which is most likely?Order Of PreferenceRequirements

E2i) strong baseii) Moderate or better LEAVING GROUPIII) Antiperiplanar H-C-C-LG

SN2i) NOT TERTIARYii) Good leaving groupiii) Good nucleophileiv) Polar solvent

E1/SN1i) STABLE carbocationii) Moderate or better leaving groupiii) Polar solvent iv) hydrogen (for E1). Hydrogen bond adjacent to the carbon bearing the leaving group

Because E1 and SN1 share the same rate determining step and both involve the production of a carbocation intermediate, it is generally understood than reactions undergo both SN1 and E1 reactions simultaneously

GRINGARD REAGENTS A Grignard reagent has the formula RMgX where X is a halogen and R is an alky or aryl(based on a benzene ring) group. - A typical gringard reagent might be CH3CH2MgBrThe Preparation Of A Grignard reagent Grignard reagents are made by adding the AKLHALIDE or halegenoalkane to SMALL BITS OF MAGNESIUM in a flask containing ethoxyethane.Bromoethane + Magnesium Ehylmagnesium Bromide

- Everything must be kept perfectly dry and in ANHYDROUS CONDITIONS because Gringard reagents react with water to produce alkanes

Grignard reagents are very STONG BASES. One of their most common uses is in their reaction wih aldehydes and ketones to form ALCOHOLS.

ISOMERISM AND STEREOCHEMISTRYIsomersIsomers are molecules with the SAME FORMULA but different structures. It is important to realise isomers can have different chemical, physical and biological properties.

Constitutional IsomersConstitutional isomers have different atoms connected to different things, they have the same structural formula though. They are also called STRUCTURAL ISOMERS.Example Compare ethers(where two hydrocarbon groups are linked by an oxygen atom, ROR)with alcohols- They have DIFFERENT CHEMICAL PROPERTIESi.e. alcohol can be oxidised (e.g. carboxylic acids) but ethers cannot be oxidised.- They have DIFFERENT PHYSICAL PROPERIESi.e. ethers are non-polar solvent and alcohols are polar solvents- They can have the same functional groups but in DIFFERENT PLACES. This would mean that they would have similar chemical properties. Like undergo the same reaction but the rate may be different. Sometimes the reaction may even be different.Stereoisomers- Stereoisomers have the same STRUCTURAL FORMULA AND CONNECTIVITIES, but the structures CANNOT BE SUPERIMPOSED UPON ONE ANOTHER They have the SAME CHEMICAL PROPERTIES They undergo the same reactions mostly with identical rates. But they have DIFFERENT PHYSICAL PROPERTIES for instance they have different MELTING AND BOILING POINTS, fluidity etc. Geometric Isomers Geometric isomers result most commonly from carbon-carbon double bonds. The important property is the INABILITY OF THE CARBON ATOMS TO ROTATE RELATIVE TO ONE ANOTHER ABOUT THE DOUBLE BOND. The molecules have identical connectivitys so they cannot be described as structural isomers- These isomers used to be called cis and trans isomers but not anymore. We now use E and Z notation:E Entgegen = oppositeZ Zusammen = togetherHOW TO DECIDE E AND Z You assign properties based on atomic number and consider each end of the double bond separately1) Take each end of the double bond separately 2) Consider the FIRST atom in each chain3) Chain which has the highest atomic number is the highest priority4) If highest priority is on the SAME SIDE of the double bond then it is Z. If it is on the OPPOSITE SIDE OF THE DOUBLE BOND then it is E

Atoms With Four Different Bonds Can Be Chiral- The centre carbon is termed the STEREOGENIC CENTRE. The bonds are arranged so that It cannot be superimposed on its own mirror image.- A pair of mirror images are called ENANTIOMERSStereogenic Centers- These have CHIRAL centres or STEREOCENTERS. Any molecule with a stereogenic centre will be chiral.- A stereogenic centre cannot be:-sp- or sp-2hybridised (MUST be sp3)-an atom with 2 identical subsitutentsProperties of EnantiomersENANTIOMERS HAVE THE SAME CHEMICAL AND PHYSICAL PROPERTIES!! The only way to distinguish them is the fact THAT THEY ROTATE POLARISED LIGHT IN DIFFERENT DIRECTIONS. And the fact that they interact with other chiral molecules differently.

ASSIGNING CHIRALITYIn order to assign the configuration as R or S, identify each of the chirality centers (most commonly an sp3C with 4 different groups attachedThen at each chirality centre Assign the priority (high=1 to low=4) to each group attached to the chirality center based on atomic number. Reposition the molecule so the LOWEST PRIORITY GROUP is away from you Determine the relative direction of the priority order of the three higher priority groups ( 1 to 2 to 3) If this is CLOCKWISE then it is the R-STEREOISOMER (Latin; rectus = right handed) If this is ANTICLOCKWISE then is the S-STEREOISOMER (Latin; sinister = left handed)

MULTIPILE STEREOGENIC CENTERS- Compounds with more than two stereocentres have more than two stereoisomers. In general n number of stereocentres gives 2n stereoisomersDIASTEREOISOMERS- These are stereoisomers that are not enantiomers . They are not exact mirror images. Anything that gives us a stereogenic centre with no mirror image must be a diastereoisomer.

MESO COMPOUNDS If we can make an enantiomer superimposable on its own mirror image by rotating it then it is NOT an enantiomer. A compound like this is called a meso compound. /- Because it is superimposable upon its mirror image, it is NOT optically active, even though it has two (or more) stereogenic carbon atoms Another to detect a meso compound is to look for a plane of symmetry within the molecule. This is particularly easy with Fischer projections. If the bottom half is a mirror image of the top half, then we have a meso compound.

CONFORMATION Although single bonds can rotate freely, not all positions are equally preferred.The simple alkane ethane is a good example of conformational analysis. Here there is just one carbon-carbon bond and the rotational structures may fall between two extremes, STAGGERED AND ECLIPSEDEclipsed groups at the front LIE DIRECTLY IN LINE WITH THOSE AT THE BACKStaggered groups at the front LIE IN LINE WITH THE GAPS AT THE BACKThe table below shows the conformations of the ethane molecule. The first view shows the molecule form the side. The hydorgens are located in the surrounding space by WEDGE(in front of the plane) and HATCHED(behind the plane) BONDS. If we rotate this structure so that carbon 1 is brought down and closer to the viewer the sawhorse projection is presented. Finally, the Newman projection is when carbon 1 is directly infront of carbon 2.

As a result of bond-electron repulsion, the eclipsed conformation is LESS STABLE than the staggered conformation. The most severe bond repulsion in the eclipsed conformation (depicted by the red arrows) and far more severe than the roughly equal bond repulsions in the staggered conformation. The staggered conformation is also a lower energy form, and lowest energy is always favoured.

SOMETIMES THE ECLIPSED CONFORMATION CAN BE MORE STABLEEthylene glycol- In the case of ethylene glycol, intramoleculer HYDROGEN BONDING is possible in the eclipsed form but not the staggered. This makes the molecule slightly more stable, by about 30 KJ mol-1, this is enough to make the eclipsed conformation MORE FAVOURABLE.

- With cyclic molecules, EQUATORIAL is more stable than ANXIAL. Things that are in the axial position are more crowded.ALKENES AND ALKYNES: Addition ReactionsAlkenes Alkenes are unsaturated hydrocarbons. They contain:- Only Hydrogen and carbon- At least one carbon-carbon double bond- Sigma bonds and Pi bonds- No lone pairs!- sp3 and sp2 hybridised carbonsThe general formula for alkenes is CnH2nGeometric Isomers- These are alkenes, they form on the basis of limited rotation around a carbon-carbon double bond.

FORMATION OF ALKENES1) Elimination from alkylhalides

2) CRACKING OF LONG-CHAIN ALKANES- We break up long chain alkanes to give a mixture of short chain alkanes and alkenes. Remember from A-level there is thermal and catalytic cracking

THE WITTIG REACTION This reaction is best for making complex alkenes- The wittig (W.ell I.snt T.his T.errifically I.nconvinient to learn G.) reaction can be used to increase the size of the carbon skeleton The reaction uses an ylid (aka ylide or phosphoranwe) -Relies on the strength of P-O bondsExample- The mechanism of the Wittig reaction is not fully established, but a simple version is given below. The initial step is the nucleophilic addition of NEGATIVELY CHARGED carbon of YLIDE onto the carbonyl carbon to give a betaine, which can cyclise to give an oxaphosphetane as an intermediate. The oxaphosphetane is decomposed to give an alkene and a phosphine oxide.- The driving force of the Witting reaction is the FORMATION OF THE HIGHYLY STABLE DOUBLE BOND BETWEEN PHOSPHORUS AND OXYGEN IN PHOSPHINE OXIDE

- Replaces the carbonyl with an alkeneREACTIONS OF ALKENES- Alkenes are ten million quantillion times more useful than alkanes. They are more reactive and the double bond is a site of extreme electron density. - We can extend the carbon skeleton using the Wittig reaction above.Addition To Alkanes This is an electrophilic mechanism and proceeds via a carbocation intermediatei) Adds positive component firstii) Generates the MOST STABLE carbocation as the intermediateTO DETERMINE THE MECHANISM1) Identify the + end of the REAGENT2) This is then attacked by the -electrons of the double bond3) To generate the most substituted carbocation4) The resulting anion (negatively charged ion) forms a bond to the carbocationSOME SPECIFIC EXAMPLES1. Alkenes Alkyl Halidese.g. using Br2: The overall transformation C=C into X-C-C-XReagent: Br2Step 1: The - electrons of the double bond act as a NUCLEOPHILE and are attracted to the bromine molecule (because of the INDUCED DIPOLE between Br-Br). This forms a BROMONIUM INTERMEDIATE

Step 2: Attack of the nucleophilic bromide form the side AWAY from the bromonium centre of the cyclic molecule in an SN2 like fashion opens up the bromonium ion to give overall trans addition.

e.g. using HBr-As with all alkenes, unsymetrrical alkenes like propenes react with hydrogen bromide in the cold.

- This shows MARKOVNIKOVS RULE This states that when a compound HX is added to an unsymmetrical alkene, the hydrogen becomes attached to the carbon on the double bond that HAS THE MOST HYDROGENS ATTACHED TO IT ALREADY. In this case the hydrogen becomes attached to the CH2 group because the CH2 group has more hydrogens than the CH group.The mechanism

2. ALKENES ALCOHOLSe.g. H2O in dilute acid (Essentially H3O+) This is a REVERSIBLE reaction.

http://www.ochem4free.info/YQcr3l7Abrssp/01-Atoms.pdfhttp://www.angelo.edu/faculty/kboudrea/general/quantum_numbers/Quantum_Numbers.htmhttp://www.smallscalechemistry.colostate.edu/PowerfulPictures/Orbitals.pdf http://www.personal.psu.edu/the1/e2.htm http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch05/ch5-5.html e2http://www.lasalle.edu/~price/Electrophilic%20Addition%20Reactions.pdf electrophilic additionhttp://www.chem.ucalgary.ca/courses/350/Carey5th/Ch14/ch14-4-6.html oxymercurication mechanismhttp://www.chem.ucalgary.ca/courses/350/Carey5th/Ch15/ch15-3-3.html alkenes to diolshttp://www.youtube.com/watch?v=WwTCf-bv1SQ epoxination of alkenes