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Chapter 19: Alkenes and AlkynesThe vast majority of chemical compounds that we know anything about and that we synthesize in the lab or the industrial plant are “organic compounds”.

The simplest organic compounds are alkanes – pure carbon and hydrogen, CnH2n+2, - but these compounds are not particularly reactive. Their octets are “satisfied” leaving them without a handle for reactivity.

From the point of view of building complex organic compounds, the alkenes (and alkynes) are much more useful.

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For example, the pigment/anti-oxidant β-carotene is generated from the isoprene unit:

aka “2-methyl-1,3-butadiene”

We can see that 8 isoprene units can be linked:

giving:

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Understanding the reactivity of a substance, such as ethylene, is a question of putting together electronic and physical characteristics of the molecule.

For ethylene, the double bond is stronger (611 kJ/mol) and shorter (113 pm) than the corresponding single bond in ethane (376 kJ/mol; 154 pm). It is also “rigid”:

This rigidity is observed in the activation energy for rotation. In single bond, Ea ≈ 12 kJ/mol. For a double bond, Ea ≈ 268 kJ/mol.

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The reason for the rigidity is that a double bond is made from two orbitals from each atom overlapping. That is, the carbon atoms are “sp2” hybridized which form the sigma bonding network in the compound. The remaining p-orbital then forms a pi orbital that provides an additional linkage between the carbon atoms:

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Things are a little more complicated – and to fully understanding bonding in organic compounds, we need to adopt a “molecular orbital theory” approach.

At its heart, M.O. Theory takes the orbitals from all of the atoms in a compound and combines them to form orbitals of the whole molecule. The one important distinction between a “valence bond” approach (or a “hybridization approach”) and M.O. Theory is that the latter combines two atomic orbitals to give two molecular orbitals – one of which is “bonding” and one of which is “anti-bonding”.

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For double bonds, the “extra” p-orbital results in a π-bonding and π-anti-bonding orbital.

The “highest occupied molecular orbital” is called the “HOMO” and the “lowest unoccupied molecular orbital” is the “LUMO”.

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Recall that the nomenclature for alkenes and alkynes must account for the presence of the double or triple bond. We do this by naming the parent hydrocarbon chain - the longest chain that incorporates the double or triple bond –and then add the suffix “-ene” or “-yne” (we replace the “-ane”).

We then number the chain so as to give the double or triple bond the lowest possible number – all things being equal.

If there are more than one double or triple bond, then we use “di-”, “tri-”, “tetra-”, etcetera.

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The internal barrier to rotation for double bonds adds an additional constraint to naming the compound. We need to distinguish between the two (or more) possible isomers.

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Recall from the last chapter that ΔG = -4 kJ/mol for this reaction. That gives a value of Keq = 5 consistent with the distribution (the difference is due to rounding). But why?

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The IUPAC-approved convention is the E,Z-system of nomenclature.

E stands for entgegen which is German for “opposite”. The highest priority groups are on opposite sides of the bond.

Z stands for zusammen which is German for “together”. The highest priority groups are on the same side of the bond.

For example:

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Recall also that organic functional groups – such as double and triple bonds – can be identified by their characteristic spectroscopic features.

For Infrared Spectroscopy (IR Spectroscopy), we have the general pattern:

There are subtleties to this.

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Stretches just below 3000 cm-1 correspond to the symmetric and anti-symmetric C-H stretches of an alkane; stretches just above 3000 cm-1 result from the symmetric and anti-symmetric C-H stretches associated with a C=C.

A C=C stretching vibration is often found in the 1640 – 1680 cm-1 region.

=C-H 3020 – 3100 cm-1 C=C 1640 – 1680 cm-1

RCH=CH2 910 and 990 cm-1 R2C=CH2 890 cm-1

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13C NMR Spectrocopy also has characteristic regions. Typical chemical shifts for alkenes are in the 110 to 150 ppm range.

Even more information can be gleaned from the 1H NMR spectrum but it is also more complicated to interpret. However, it does have broad ranges:

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In dealing with “inorganic reactions” – the reactions of salts and such – there are a serious of descriptions for different types of reactions such as “double displacement” or “acid-base”.

Organic reactions also have classifications based on the mechanism and/or the type of products produced.

In all cases, organic reactions can generally be described by the rearrangement of electron pairs to give products which is the basis for our mechanisms.

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Addition Reactions:

These occur when two or more molecules come together, resulting in a single new product with no leftover atoms.

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Elimination Reactions:

In a sense, an elimination reaction is simply the reverse of an addition. That is, a single small molecule is released and the resulting residual organic fragment regenerates its double bond.

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Substitution Reactions (SN1 and SN2):

These require the presence of a “leaving group” which is a group that can be displaced to give an ionic or neutral species. They require that the incoming group has a similar charge to the leaving group and is also a good “nucleophile” – a species that interacts with a positively charged carbon.

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Rearrangement Reactions:

These occur when a molecule undergoes an internal reorganization of its atoms with no net gain or loss. This might involve the interconversion of isomers (i.e. cis to trans) but could also involve the shifting of groups within a molecular structure or the simple cycling of electrons.

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Visualizing the mechanism of an organic reaction is not actually possible – in an experimental sense. At best, we can measure some of the intermediate species along the way. But putting it together leads to an understanding of the patterns and processes that drive chemistry.

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Many organic reactions involve polar groups –nucleophiles and electrophiles – and essentially are “acid-base” reactions where electrons move in pairs to form and break bonds.

When electrons move in pairs, bonds break heterolyticallyand form in a heterogenic manner.

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In radical reactions, the bond breaking and making proceeds via a “one electron” process – similar to electrochemistry – where each partner ends up with one electron from the bonding pair. Bond breaking is “homolytic” (the bond undergoes “homolysis”) and bond making is “homogenic”.

The reaction products may be “free radicals” depending upon the rate constant for their quenching.

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The fundamental characteristic of all polar reactions is that electron-rich sites (nucleophiles) react with electron-poor sites (electrophiles) in another molecule (or even within the same molecule) because the negatively charged region of one molecule is attracted to the positively charged region of another. Each of these could be considered a Lewis Acid.

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A typical reaction – and one of the simplest – is the addition of HX to an alkene. For example:

HCl + H2C=CH2 → H3C-CH2Cl

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The reaction proceeds through the “π-bonding electrons” because they are more accessible:

The fact that the π-bond is both “electron-rich” and “electron accessible” is consistent with the mechanism.

Also, the +ve charged carbon (with only 6e-) is called a “carbocation” and is unstable, so very reactive.

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From an energy point of view:

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With ethylene, it doesn’t make any difference which way around the HX adds as the product is the same. However, with an unsymmetrical alkene, we can get different products:

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These reactions are “regioselective” and consistent with Markovnikov’s rule which says “those that has, gets”. Actually, it says “that the more stable carbocation will form on the carbon with the greatest number of alkyl groups attached.”

When it’s a tie, both compounds form:

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The stability of carbocations:

Predict the products:

CH3CH2CH=CH2 + HCl →

CH3CH=C(CH3)2 + HI →

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Addition of H2O to Double Bonds: Hydration

Water consists of “H+ + OH-” – which means that it should add to a double bond in the same way that HX does.

The mechanism is similar to the reaction with HX but there is an added step as it is OH2 that acts as the nucleophile. It also follows Markovnikov’s Rule.

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But the subtle difference between HX and H2O addition is “heat”. The hydration requires harsh conditions – for example, H3PO4 and 250°C. As a consequence, chemists have continually searched for better ways to do a hydration.

Oxymercuration-demercuration (Markovnikov addition)

Hydroboration (anti-Markovnikov addition)

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Further, hydration is matched with dehydration – the formation of an double bond by the removal of water. There are various catalysts that will do this – the most common is sulphuric acid.

Many of the reactions that alkenes do in one direction can be reversed – such as “halogenation” and “hydrogenation”.

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Addition of X2 to Double Bonds: Halogenation

Again, although a little less obvious, Cl2 and Br2 can be thought of as “Cl+ + Cl-” and “Br+ + Br-”. These are not real ions but the halogens behave as if they are.

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Mechanistically:

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Addition of H2 to Double Bonds: Hydrogenation

Hydrogenation involves the reduction of a double bond when exposed to an atmosphere of hydrogen in the presence of a catalyst. Book says “heterogeneous” process in that the catalyst is a “solid” and the reaction happens at the surface. Not always.

Reduction in organic chemistry refers to an increase in electron density on carbon atoms by forming a C-H bond or breaking a C-O, C-X, or C-N bond.

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Oxidation of Double Bonds: Hydroxylation and Cleavage

Hydroxylation is the addition of –OH to each end of a C=C bond. There are a number of reagents that can accomplish this but KMnO4 (in basic conditions) is representative:

Oxidation in organic chemistry refers to decreasing the electron density on C atoms by breaking C-H bonds or by forming a C-O, C-X, or C-N bond.

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Under acidic conditions, KMnO4 leads to cleavage of the double bond.- If there are no hydrogens on either end, then the

products are ketones.

- If there is a hydrogen, then one of the products is a carboxylic acid.

- If the double bond is terminal (=CH2), then CO2 is produced.

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Free Radical Reactions: Polymerization

These are a “unique” class of organic reactions. Industrially, these are hugely important.

Three steps:1) initiation: a radical species is generated

followed by

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2) Propogation: free radical addition of monomers

3) Termination: coupling of two free radicals to give a C-C bond

2R-CH2CH2. → R-CH2CH2-CH2CH2-R

Many different types of monomers:

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Alkynes are hydrocarbons with a triple bond – but we can think of them as a “two double bonds”. Shorter and stronger, the electrons are more exposed:

Recall that the spectroscopy of these compounds must take into account the triple bond:

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Generally, the reactivity is that of two double bonds:

Addition of HX:

Addition of X2:

Addition of H2O:

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Addition of H2:

Alkynes can be converted to alkenes by the addition of 1 equivalent of hydrogen or all the way to alkanes by the addition of 2 equivalents.

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Summarizing: