ALKENES, ALKYNES AND AROMATIC COMPOUNDS

ASSIGNMENT
QUESTION: Give the physical, chemical and uses of the following.
A. Alkene
B. Alkyne
C. Aromatic compounds


ANSWERS
1. Alkene
Physical properties
a. Physical state:  Ethene, propene, and butane exist as colourless gases. Members of the 5 or more carbons such as pentene, hexane, and heptene are liquid, and members of the 15 carbons or more are solids.
b. Density: Alkenes are lighter than water and are insoluble in water due to their non-polar characteristics. Alkenes are only soluble in nonpolar solvents.
c. Solubility:  Alkenes are virtually insoluble in water, but dissolve in organic solvents. The reasons for this are exactly the same as for the alkanes.
d. Boiling points: The boiling point of each alkene is very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and the various butenes are gases at room temperatures. All the rest that you are likely to come across are liquid. Boiling points of alkenes depends on low molecular mass (chain length). The more intermolecular mass is added, then the higher the boiling point. Intermolecular forces of alkenes get stronger with increase in the size of the molecules.
e. Melting points: melting points of alkenes depends on the packaging of the molecules. Alkenes have similar melting points to that of alkanes, however in cis isomers molecules are package in a u-bending shape, therefore, will display lower melting points to that of the trans isomers.
f. Polarity: chemical structure and functional groups can affect the polarity of alkenes compounds. The sp2 carbon is much more electron-withdrawing than the sp3 hybridized orbitals, therefore, creates a weak dipole alone the substituent alkenly carbon bond. The two individual dipoles together form a net molecular dipole. In trans-substituted alkenes there is a net dipole, therefore contributing to higher boiling in cis-isomer than trans-isomer.
Alkenes Chemical Properties
Although the double bond between two carbon atoms is stronger link than a single bond, it is not twice as strong, (i.e. the second bond formed between the carbon atoms is weaker than the first). Thus, the second bond is more vulnerable to attack by suitable reagents, even under fairly mild conditions. Thus, the reaction of this second bond tends to be addition reactions, where the unsaturated carbon atoms become saturated. The alkenes are much more reactive than alkanes.
Combustion of Alkenes
The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water,. For example, ethene burns as follows:


C2H4   +   3 O2   ==>   2 CO2   +   2 H2O      
Addition Reactions across the Double Bond
Because the alkenes are unsaturated hydrocarbons, their most important reactions are addition reactions across the double bond.
The alkenes are readily oxidised by potassium permanganate to form glycols. For example, ethene is oxidised to ethylene glycol.


3 H2C=CH2   +   2 KMnO4   +   4 H2O    

==>     2MnO2  +  2KOH   +   CH2OHCH2OH

Ethylene Glycol
During the oxidation of alkenes, the purple colour of the permanganate solution disappears and the reaction constitutes a test, known as Baeyer's Test, to detect unsaturation in any compound.
Addition of Hydrogen
The alkenes are readily reduced by the addition of hydrogen across the double bond to form alkanes (i.e. reduction of alkenes). For example, when alkenes are passed over a nickel catalyst at 150oC, the alkene is reduced to an alkane.


H2C=CH2   +   H2      ==>     CH3CH3

Ethene                                       Ethane
Addition of Halogen
Halogens readily add across the double bond of the alkenes to form dihalides

H2C=CH2   +   Cl2       ==>     CH2ClCH2Cl    

Ethene                          DiChloroEthane
H2C=CH2 + Br2 ==> H2Br CH2Br
Ethene DiBromoEthane
The decolourisation of bromine is a second test for an unsaturated organic compound.
Addition of Hydrogen Halide
Hydrogen halides readily add across the double bond of the alkenes to form alkyl halides the reactivity of ethene, with the halogen acids is in the order


HI >> HBr > HCl    
Thus, ethene reacts readily with hydrogen iodide and with hydrogen bromide at room temperature to form ethyl iodide and ethyl bromide, respectively.

H2C=CH2     +    HI      ==>     CH3CH2I
Ethyl Iodide  
With ethene, the hydrogen atom of the halogen acids can add to either carbon atom to yield bromoethane.
However, with higher members of the ethene series, the orientation of the addition of asymmetric molecules across the double bond is governed by the Markownikoff Rule.
Addition of Water
Water can add across the double bond of the alkenes to form aliphatic alcohols. This is hydration reaction is catalysed under a number of different conditions.
When ethylene and steam are heated (i.e. at 300oC) under high pressure (i.e. at 70 atm.) in the presence of a catalyst (i.e. phosphoric acid, on a silica support), ethanol is formed.
H2C=CH2   +   H2O       ==>     C2H5OH
Reaction with Sulphuric Acid
Similarly, fuming sulphuric acid absorbs ethylene at room temperature to form ethyl hydrogen sulphate, with much evolution of heat.
C2H4   +   H2SO4 ==>     C2H5.HSO4      
If this is treated with water and warmed, ethanol is formed.  
C2H5.HSO4   +   H2O     =heat=>     C2H5OH + H2SO4        
Polymerisation Reactions due to the Double Bond
When ethylene is heated under great pressure in the presence of a catalyst a large number of the molecules combine to form polythene, (C2H4)n, (i.e. Polyethylene). This particular kind of reaction is called an addition polymerisation and the mechanism by which it takes place is a reaction is a free radical chain reaction. The overall reaction is

  n (C2H4)         ==>             (C2H4)n
Ethene                          Polythene
Uses of alkenes
 Alkenes are extremely important in the manufacture of plastics. All plastics are in some way related to alkenes. The names of some plastics (Polythene or Poly Ethene, Polypropene), relate to their alkene partners. Plastics are used for all kinds of tasks, from packaging and wrapping, to clothing and outdoor apparel.
 Lower alkenes are used as fuel and illuminant. These may be obtained by the cracking of kerosene or petrol.
 For the manufacture of a wide variety of polymers, e.g., polyethene, polyvinylchloride (PVC) and Teflon etc.
 As raw materials for the manufacture of industrial Chemicals such as alcohols, aldehydes, and etc.
 Besides, alkenes also used for artificial ripening of fruits, as a general anesthetic, for making poisonous mustard gas (War gas) and ethylene-oxygen flame.
2. ALKYNES
Physical properties of alkynes
Physical state
The first three members (ethyne, propyne and butyne) are colourless and odourless gases. Due to the presence of phosphine as an impurity ethyne (acetylene) has garlic smell. The next eight members are liquids, and higher members are solids under normal conditions of temperature and pressure.
Solubility
Alkynes are insoluble in water, but are fairly soluble in organic solvents such as, alcohol, ether, acetone etc.
Melting and boiling points
The melting and boiling points of alkynes increase with molecular mass. Melting boiling points of some alkynes are,
Ethyne/acetylene (CH º CH), - 83oC or 190 K
Propyne (CH3-CºCH), - 27oC or 246 K
1 - Butane (CH3-CH2-CºCH), 8oC or 281 K
2 - Butane (CH3-CºC-CH3), 29oC or 302 K
1 - Pentyne (CH3-CH2-CH2-CºCH) 48oC or 321 K
2 - Pentyne (CH3-CºC-CH2-CH3) 55oC or 328 K
Chemical properties of alkynes
Alkynes contain a triple bond ( ). A triple bond has one and two bonds.
Some characteristic reactions of alkynes are,
Combustion
Alkynes burns in air or oxygen with smoky flame.

Electrophilic addition reactions
Carbon-carbon triple bond, C=C, is a combination of one and two bonds. Alkynes give electrophilic addition reactions as they show reactivity due to the presence of bonds. This property is similar to alkenes but alkynes are less reactive than alkenes towards electrophilic addition reactions due to the compact CC electron cloud. Some typical electrophilic addition reactions given by alkynes are:
Addition of hydrogen
An alkyne reacts with hydrogen in the presence of catalyst (Pt or Ni) at 250°C, first forming alkenes and finally alkane.

For example, ethyne gives ethane in two steps.

Ethyne etheneethane
Ethane is obtained in good yields if hydrogenation is done with a calculated amount of hydrogen in the presence of palladium or barium sulphate.
Propyne gives,


Addition of halogens
Alkynes react with halogens (Cl2 or Br2) in the dark, forming dihaloalkenes first and finally tetrahaloalkanes. The reaction gets accelerated in the presence of light or halogen carriers.
RCCH                   RCX=CHX     RCX2CX2
alkyne dihaloalkene tetrahaloalkane
For example, ethyne (acetylene) with chlorine gives,


The order of reactivity is Cl2 > Br2 > I2.
Addition of halogen acids
Alkynes react with halogen acids according to the Markownikoff's rule i.e. the carbon atom carrying the least number of hydrogen atoms will have the negative part of the addendum attached to it.

For example, ethyne (acetylene) with HBr gives,

Addition of hypochlorous acid
Alkynes react with hypochlorous acid according to the Markownikoff's rule.

For example, ethyne with HOCl gives,

Dichloroethanal
In the presence of peroxides the addition of HBr takes place according to the anti-MarkowniKoff's rule.
Addition of sulphuric acid
Alkynes add up two molecules of sulphuric acid. For example, ethyne gives

Nucleophilic addition reactions
Alkynes also give the following nucleophilic addition reactions.
Addition of water
In the presence of sulphuric acid (42%) and 1 % mercuric sulphate at 60°C, alkynes add on one water molecule to give aldehydes or ketones. For example,

Alkyne ketone
Ethyne gives ethanal and propyne gives acetone.

ethyne (acetylene) ethanal (acetaldehyde)


Addition of HCN
Alkynes add one molecule of HCN in the presence of Ba(CN)2. For example,

Ethyne gives

ethyne vinyl cyanide

Addition of ozone
Ozone adds up across the triple bond to give ozonides. After hydrolysis, ozonides give diketones and carboxylic acids.

Ethyne gives glyoxal and formic acid,

glyoxal formic acid

Substitution reactions
Due to their acidic nature, alkynes form metallic salts called alkynides e.g., sodium, silver and copper(ous) salts. Examples are,


Acidic hydrogen in 1-alkynes
Hydrogen atoms in ethyne and 1-alkynes, linked to the carbon atom having a triple bond on it, are acidic in nature. For example, ethyne (acetylene) is a weak acid: weaker than water but stronger than ammonia. This may be explained as follows:
The -electrons are more weakly bound than electrons. Thus, in those compounds containing carbon-carbon double or triple bonds, the electron density around such carbon atoms will be lesser than the carbon atoms linked only through bonds. Thus, electronegativity of differently hybridized carbon atoms will follow the order,sp > sp2 > sp3
i.e., the electronegativity will increase with the s character in the hybrid orbitals. This increase in the electronegativity of an alkyne carbon, (relative to the carbon atoms in alkenes and alkanes) will polarize the C-H electron bond towards carbon and facilitate the release of proton(s). Accordingly the acid strength of hydrogens will follow the order, Alkynes > Alkenes > Alkanes.
The stabilities of the anion left after the removal of proton, i.e. carbanions follow the order,
RC C- > RCH = CH- > R-CH2-CH2-
Thus, the acid strength follows the order, HC CH > H2C = CH2 > H3C-CH3
Compared to the organic acids e.g...Ethanoic acid (CH3OOH), ethyne is about 1020 time less acidic, while ethane is 1040 times less acidic.
Polymerization
On heating alkynes undergo polymerization in the presence of catalyst. The nature of products depends upon the conditions. For example,
• When ethyne (acetylene) is passed through a hot copper tube, it polymerizes to benzene.

ethyne benzene benzene
• When passed through a solution of cuprous chloride in ammonium chloride, ethyne undergoes linear polymerization.
• Vinyl acetylene with hydrogen chloride gives chloroprene (2-chlorobuta-1,3-diene), which readily polymerizes to give neoprene (a synthetic rubber)

Oxidation
Oxidation of alkynes gives mono or dicarboxylic acids.
For example,
• Alkaline KMnO4 oxidises ethyne to oxalic acid.
Oxalic acid (ethanedioic acid)
• With chromic acid, ethyne gives acetic acid.
Ethyneethanoic acid (acetylene) (acetic acid)
Homologues of ethyne on oxidation with alkaline KMnO4 give mixture of acids. During oxidation, rupture takes place at the triple bond.

3. AROMATIC COMPOUNDS

Physical properties of aromatic compounds
 Non polar
 good industrial solvents for other non polar compounds
 Insoluble in water
 liquid at room temperature

Chemical properties of aromatic compounds
 They undergo electrophilic substitution reactions.
 They undergo mono substitution of derivatives of benzene.
 They undergo addition reactions under drastic condition.
 They undergo side-chain substitution reaction.
 They undergo oxidation reaction.

USES OF AROMATIC COMPOUNDS
 They occur naturally in compounds like DNA and within some amino acid that make up protein and chlorophyll.
 They make up the heme groups which helps the blood cells carry oxygen.
 They are the spicy compounds in hot pepper, ginger and other black compounds
 Health: aromatic compounds are used in drugs formation and purifications
 Automotives: car bodies, bumpers, lightening, dashboard, seat, upholstery, fuel system, bonnets are made from products derived from aromatic compounds.
 Clothing: aromatic compounds are used in textile industry for manufacturing of clothes.
 Sports equipments: aromatic compounds are used in the productions of sport equipments e.g football, swim suits etc


REFERENCES
1. Vollhardt, K,P.C & shore, N. (2007). Organic chemistry  (5th ED) new York: W.H freemen (453-454)
2. Vollhardt, K,P.C & shore, N. (2007). Organic chemistry: study guide and solution Manuel (5th ED.). new York: W.H Freeman (200-202)
3. https://www.ucc.ie/academic/chem/dolchem/html/dict/alkenes.html