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ALKENES Definition of alkenes Alkenes are hydrocarbons that contain carbon carbon double bond (C=C). The general formula of alkenes are CnH2n in comparison to alkanes with general formula CnH2n+2. Alkanes are saturated with hydrogens, while alkenes are two hydrogen less than alkanes. Thus it is known as unsaturated hydrocarbons. For example:

What Are Alkenes?

Alkenes, also known as olefins, are organic compounds that consist of carbon and hydrogen atoms with one or more carbon-carbon double bonds in their chemical structure. Alkenes are unsaturated hydrocarbons. They are hydrocarbons because they are made of only carbon and hydrogen atoms, and they are unsaturated because they have one or more double bonds in their chemical structure.

Let's take a look at the general chemical structure of an alkene (RC=CR'). We can see two carbon atoms that are bonded together with a double bond, and they can be illustrated in two ways. The first way is one where the carbon atoms are shown as the letter C with lines indicating the bonds. The other way is to represent the carbon atoms with a bend in the line. These carbon atoms have R and R' side groups attached to them. These side groups can be any group of carbon and hydrogen atoms that are bonded together.

Let's compare the chemical structures of two alkenes, isobutylene and 1-pentene.

We can see that they are alkenes because they have carbon-carbon double bonds.

Besides that, notice their names; do you see anything in common? They both end with -ene. Alkenes are conventionally named with the suffix -ene.

These two examples are chemical structures of alkenes that look like carbon chains that can have branches. Now, let's imagine attaching these carbon atoms to each other and closing them to form a ring. Here, we have another type of alkene called a cycloalkene, which has rings of carbon atoms bonded together, and within the rings, it contains one or more double bonds.

Properties of Alkenes: What does an alkene look like? Are they solids, liquids, or gases? The answer is: all of the above. Earlier, we talked about alkenes as chains of carbon and hydrogen atoms bonded together. The physical state (solid, liquid, or gas) of alkenes is dependent on the number of their carbon atoms.

Simple alkenes, like ethene and propene, that have two to four carbon atoms generally exist as gases.

Alkenes with five to sixteen carbon atoms are generally liquids. Alkenes having seventeen or more carbon atoms are generally waxy solids, like paraffin wax, which is used to make candles.

While the physical state of alkenes varies depending on the number of carbon atoms, what alkenes have in common is that they are both insoluble in and less dense than water. However, they are, in general, soluble in organic solvents, so they will dissolve if you try mixing an alkene with an organic solvent like acetone or benzene.

Physical Properties of Alkynes: 

The properties of alkynes pretty much follow the same pattern of those of alkanes and alkenes.

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Alkynes are unsaturated carbon that shares a triple bond at the carbon site

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All alkynes are odourless and colourless with the exception of ethylene which has a slight distinctive odour.

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The first three alkynes are gases, and the next eight are liquids. All alkynes higher than these eleven are solids

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Alkynes are slightly polar in nature

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The boiling point and melting point of alkynes increases as their molecular structure grows bigger. The boiling point increases with increase in their molecular mass

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Also, the boiling points of alkynes are slightly higher than those of their corresponding alkenes, due to the one extra bond at the carbon site.

Properties of Aromatic Hydrocarbons: 

Properties of Aromatic Hydrocarbons include that their major sources are Petroleum and coal. They are well known for their exceptional physical and chemical properties. Poly-aromatic hydrocarbons are defined as aromatic compounds with more than one benzene. When they include in atmospheric pollution then it is known as carcinogenic in nature.

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Aromatic compounds also include amino acids and precursors to nucleotides. Which are soluble in water they are known as non-polar hydrocarbons. These hydrocarbons cannot form ions or H-bonds with water molecules. They are usually unreactive because of extra stability and for many organic and inorganic reactions it is widely used as an inert solvent.

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The ratio for carbon-hydrogen is high. They born with sooty yellow flame because of the presence of high carbon content.

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They go through electrophilic substitution reactions and nucleophile aromatic substitution.

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Hydrocarbons which have multiple bonds are unsaturated in nature like alkenes and alkynes. They tend to give addition reactions due to this unsaturation.

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Due to resonance and give characteristic electrophilic substitution reactions aromatic hydrocarbons are stable. The carbon ring acts as a nucleophile in these reactions and to form a substituted product an electrophile attack on benzene.

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With the coming electrophile, one of the H-atom of a ring is substituted because of this the product also holds its stability and aromatic in nature. On the opposite side in the addition reactions, aromatic compound may lose their aromaticity so they do not prefer to give such reactions.

Video on Preparation and Properties of Hydrocarbons

Uses of Aromatic Hydrocarbons 

In several industries, aromatic hydrocarbons have wide applications. For example, for model glues, toluene is used as solvent while naphthalene is used as mothballs.

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For manufacturing of dyes, explosives, and drugs, Phenanthrene is an intermediate product which has a different synthetic process. Trinitrotoluene (TNT)

or 2, 4, 6 trinitrotoluene is an important aromatic compound which is mainly used as explosive along with the preparation of explosive. 

1, 2 benzenediols or pyrocatechol is advertised as catechol which is one of the most important components of a photographic developer.

Structure and bonding  

"Unsaturated" hydrocarbons Contain at least one C=C bond C-C C=C

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Bond strength 356 kJ mol-1 636 kJ mol-1

Each carbon atom of the C=C bond is sp2 hybridized The sp2 hybridized carbon centres are planar with bond angles of 120 The double bond is composed of overlap of two sp2 hybridized orbitals to form a sigma (s ) bond and overlap of two p orbitals to form a pi (p ) bond.

p bonds are weaker and more reactive than s bonds as a result of less efficient orbital overlap. s bond p bond

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Bond length 154 pm 133 pm

Bond strength 356 kJ mol-1 280 kJ mol-1

p bond electron density concentrated above and below plane of molecule Rotation about a carbon-carbon double bond is NOT possible, as this requires the p bond to break (280 kJ mol-1 of energy).

Nomenclature and Preparation of Alkenes : Nomenclature Alkenes are necessary chemical compounds that exist in our homes and daily lives.

However, we have never really given much thought to it. Let’s discuss some very common example of how alkenes have found their way to our home. We use different forms of plastic bottles and containers of plastics for storage purpose. Several times we have come across alkenes like ethane and propene. However, are you aware of the fact that these simple alkenes are important components of polystyrene and polyvinyl chlorides (PVC)? These chemical compounds are necessary for manufacturing many types of plastics. Alkene such as 1, 3- butadiene is used for making rubber products. In this topic, we will study the different methods of preparation of alkenes, a simple but necessary group of hydrocarbons.

Rules:    

Stem derived from the number of carbon atoms in longest chain which includes the double bond Ending is ene Give double bond lowest possible number (always between carbons 1, 2 in cyclic compounds) Name the substituents as usual Examples:

IUPAC name of CH2=CH2 is ethylene, and of CH2=CHCH3 is propylene Constitutional isomers are possible, e.g. C4H8, (write names)

Stereoisomers are also possible as there is no rotation about the double bond.

Stereoisomers of this sort are called diastereoisomers or diastereomers; they have different physical properties.

m.p. = -139 C b.p. = 4 C (Z)-2-Butene

m.p. = -106 C b.p. = 1 C (E)-2-Butene

Requirements for Alkene Stereoisomers 

Each end of the double bond must have groups that can be distinguished from one another.

In general, Example:

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Z/E determined by assigning a priority to each of the pairs of groups on each carbon of the double bond The higher the atomic number of the atom attached, the higher the priority If identical atoms are attached to each carbon of C=C, work outwards along the chain until the first point of difference is reached If groups of high priority are on the same side of the double bond the alkene is denoted (Z) from the German zusammen "together", if groups of high priority are on the opposite side of the double bond the alkene is denoted (E) from the German entgegen "opposite".

Example:

Preparation of alkenes  

Ethylene and propylene from refining Lab methods for other alkenes Reactions of Alkenes

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Non-polar, hydrophobic compounds

General type of reaction is an electrophilic addition reaction. The electrons of the p bond are available for reaction, which has the net effect of replacing a p bond with a s bond and, therefore, is energetically favorable.

1. Hydrogenation addition of hydrogen.  

Require a catalyst (Pt/C or Pd/C) to help break strong H-H bond Form a saturated hydrocarbon from an unsaturated one Example:

2. Halogenation   

No catalyst required for addition of Cl2 or Br2 Addition of I2 is reversible and F2 also reacts with C-H bonds so not so useful Solvent is usually CCl4 Example: 3. Hydrohalogenation

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HCl, HBr, HI Solvent is usually anhydrous CCl4 Example: 4. Hydration

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Effectively addition of H-OH but require H+ catalyst Use a dilute aqueous solution of H2SO4 Example: Classification of Reactions It is convenient to classify reactions according to the overall type addition, substitution, elimination and, where appropriate, the nature of the reagent which

first interacts with the organic substrate. We need to define two new terms, which describe this nature: electrophile and nucleophile. These terms are complimentary, just like acid and base. Note the terms acid/base may also apply but generally have more limited use in organic chemistry.

Electrophile: A species that seeks and an electron pair Nucleophile: A species that supplies and an electron pair For example: H+ + H2O ÂŽ H3O+ H+ is the electrophile and H2O the nucleophile Curly Arrow Notation Curly arrow notation is used to indicate the movement of electrons in a chemical reaction and thus shows how bonds form and break. A "curly arrow" indicates the movement of an electron pair. It starts at the electron pair that moves and ends at the atom to which the electron pair has moved.

For example: Here the lone pair on the oxygen is forming a new O-H covalent bond with the proton. and similarly Here the O-H bond is breaking and the electron pair from the bond ends up as pair on the oxygen atom of water.

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Mechanism of the Electrophilic Addition Reaction The addition of HX to a symmetric alkene gives only one possible product.

If an unsymmetrical alkene is used, there are two possible products, one of which dominates the product mixture.

This was observed before it was understood and formulated as Markovnikovs Rule: The hydrogen of an unsymmetrical reagent adds to the carbon of the double bond that has the greatest number of hydrogens already directly attached to it.

The explanation of this selectivity (regioselectivity) is as follows: o

The reaction is stepwise Reactants ® Intermediate ® Product

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The hydrogen of HX, as H+, attacks the p bond of the alkene to form a new s bond and a charged intermediate called a carbocation

The carbocation reacts with the negative part of the reagent (Br �) to form a neutral compound.

Any other nucleophile may also react.

The selectivity results from the most stable carbocation intermediate being the one which forms: tertiary > secondary > primary

Hence: The evidence for the mechanism of the reaction is as follows:

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The reaction is selective The rate of reaction increases with acidity of HX (HI > HBr > HCl > HF) The rate of reaction increases with a greater number of alkyl groups on the alkene

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The reaction in the presence of HBr, H2O and NaCl gives three products

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There is no reaction in the absence of H+

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Summary:        

Alkenes are hydrocarbons that contain carbon carbon double bond (C=C).

In IUPAC system the alkenes are named by changing the suffix of alkanes –ane to – ene.

In common system the alkenes are named by changing the suffix of alkanes from – ane to –ylene.

The carbon atoms that create double bond, are sp2 hybridized.

Due to the restricted rotation around the double bond, geometrical isomerism is possible in alkenes.

It sparingly dissolved in water and readily dissolved in organic solvent.

One of the common synthesis of alkene is by the elimination reaction from alkyl halide, alcohol and similar compound.

Alkenes perform many addition reactions via the mechanism of electrophillic addition.