All About Alkenes

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Alkenes

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 temperature. All the rest that you are likely to come across are liquids.

In each case, the alkene has a boiling point which is a small number of degrees lower than the corresponding alkane. The only attractions involved are Van der Waals dispersion forces, and these depend on the shape of the molecule and the number of electrons it contains. Each alkene has 2 fewer electrons than the alkane with the same number of carbons.

Solubility

Alkenes are virtually insoluble in water, but dissolve in organic solvents.

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February 1, 2010 Posted by | Uncategorized | Leave a comment

THE REACTIVITY OF ALKENES

THE REACTIVITY OF ALKENES

(1) Hydrogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes with hydrogen in the presence of a metal catalyst such as platinum, nickel or either palladium.  This is called hydrogenation. It includes the manufacture of margarine from animal or vegetable fats and oils.

(2) Halogenation of alkenes

The reaction of the carbon-carbon double bond in alkenes such as ethene with halogens such as chlorine, bromine and iodine. This is called halogenation. Alkenes react rapidly with chlorine or bromine in CH2Cl2 at room temperatura to form vicinal dihalides.

Reactions where the chlorine or bromine are in solution (for example, “bromine water”) are slightly more complicated and are treated separately at the end.

Chlorine reacts faster than bromine, but the chemistry is similar. Iodine reacts much, much more slowly, but again the chemistry is similar. You are much more likely to meet the bromine case than either of these.

(3) Hydrohalogenation of Hydrocarbons

Unsaturated hydrocarbons (alkenes and alkynes) react with hydrogen halides (HX). In addition reactions producing halogenated compounds .

Markovnikov’s Rule:

–       In additions of HX to unsymmetrical alkenes, the H+ of HX goes to the double-bonded carbon that already has the greatest number of hydrogens

–       Hydrogen halides are polarized molecules, which easily form ions. Hydrogen halides also add to alkenes by electrophilic addition.

Anti-Markovnikov’s Rule:

–       Hydrogen bromide can also be added to an alkene in an anti-Markovnikov fashion.

–       In anti-Markovnikov additions, the hydrogen atom of the hydrogen halide adds to the carbon of the double bond that is bonded to fewer hydrogen atoms.

–       For this to result, the reaction must proceed by a noncarbocation intermediate; thus in the presence of peroxide, the reaction proceeds via a free-radical mechanism, with the major product being generated from the more stable free radical.

February 1, 2010 Posted by | Reactivity Of Alkenes | Leave a comment

IUPAC NOMENCLATURE OF ALKENES

To name alkenes:

1. Find the longest chain containing the alkene

2. Number the chain, giving the double bond the lowest possible number.

3. For cycloalkenes, begin numbering at the double bond and proceed through the double bond in the direction to generate the lowest number at the first point of difference.

4. Assign stereochemistry using the E-Z designation

February 1, 2010 Posted by | Uncategorized | Leave a comment

Experiment : Combustion test of saturated and unsaturated hydrocarbons

Procedure :
1. 1mL of cyclohexane and cyclohexene are placed in two separate evaporating dishes.
2. Both compound are ignited simultaneously with a burning wood splint.
3. The colour intensity of the flame and the soot given off are compared.
Note : This test is not carried out with toluene because of its carcinogenic properties.

Result:
1. Cyclohexane produces less colour intensity and less soot given off during combustion test.
2. Cyclohexene produces more colour intensity and more soot is given off.

Discussion :
1. Cyclohexane is a saturated hydrocarbon and cyclohexene is an unsaturated hydrocarbon due to the presence of double bond.
2. Both hydrocarbons produce carbon dioxide and water during combustion test. When the oxygen is limited, the product will be carbon monoxide and water.
3. Cyclohexene burns and produces more soot because of the higher percentage of carbon compared to cyclohexane.

February 1, 2010 Posted by | Experiment | Leave a comment

IUPAC nomenclature of organic chemistry

A systematic method of naming organic chemical compounds as recommended by the International Union of Pure and Applied Chemistry (IUPAC). Ideally, every organic compound should have a name from which an unambiguous structural formulae can be drawn. There is also an IUPAC nomenclature of inorganic chemistry. See also phanes nomenclature of highly complex cyclic molecules.

The main idea of IUPAC nomenclature is that every compound has one and only one name, and every name corresponds to only one structure of molecules (i.e. a one-one relationship), thereby reducing ambiguity.

For ordinary communication, to spare a tedious description, the official IUPAC naming recommendations are not always followed in practice except when it is necessary to give a concise definition to a compound, or when the IUPAC name is simpler (viz. ethanol against ethyl alcohol). Otherwise the common or trivial name may be used, often derived from the source of the compound.

The IUPAC Systematic Approach to Nomenclature

A rational nomenclature system should do at least two things. First, it should indicate how the carbon atoms of a given compound are bonded together in a characteristic lattice of chains and rings. Second, it should identify and locate any functional groups present in the compound. Since hydrogen is such a common component of organic compounds, its amount and locations can be assumed from the tetravalency of carbon, and need not be specified in most cases.
The IUPAC nomenclature system is a set of logical rules devised and used by organic chemists to circumvent problems caused by arbitrary nomenclature. Knowing these rules and given a structural formula, one should be able to write a unique name for every distinct compound. Likewise, given a IUPAC name, one should be able to write a structural formula. In general, an IUPAC name will have three essential features:

A root or base indicating a major chain or ring of carbon atoms found in the molecular structure.
A suffix or other element(s) designating functional groups that may be present in the compound.
Names of substituent groups, other than hydrogen, that complete the molecular structure.

As an introduction to the IUPAC nomenclature system, we shall first consider compounds that have no specific functional groups. Such compounds are composed only of carbon and hydrogen atoms bonded together by sigma bonds (all carbons are sp3 hybridized).

February 1, 2010 Posted by | IUPAC Nomenclature | Leave a comment

Alkene Nomenclature

Alkenes represent one of the most common functional groups in organic chemistry. An alkene contains only carbon and hydrogen (a hydrocarbon) and contains at least one double bond (termed an unsaturated hydrocarbon). Alkenes have the general formula CnH2n, thus, an alkene with 10 carbons (n = 10) will have 2(10) = 20 hydrogens, or the molecular formula C10H20; each double bond therefore contributes one degree of unsaturation.

The root, or parent name for an unbranched alkene is taken directly from the number of carbons in the chain according to a scheme of nomenclature established by the International Union of Pure and Applied Chemistry (IUPAC), as described previously for alkanes.

To name alkenes:

1. Find the longest chain containing the alkene

The IUPAC name for an alkene is constructed of two parts: 1) a prefix (meth… eth… prop…, etc.) which indicates the number of carbons in the main, or parent, chain of the molecule, and 2) the suffix …ene to indicate that the molecule is an alkane.

For branched-chain alkanes, the name of the parent hydrocarbon is taken from the longest continuous chain of carbon atoms containing the double bond.

2. Number the chain, giving the double bond the lowest possible number.

Numbering of the carbons in the parent chain is always done in the direction that gives the lowest number to the double bond, or, the lowest number at the first point of difference. If there are different substituents at equivalent positions on the chain, the substituent of lower alphabetical order is given the lowest number.

If the same substituent occurs more than once in a molecule, the number of each carbon of the parent chain where the substituent occurs is given and a multiplier is used to indicate the total number of identical substituents; i.e., dimethyl… trimethyl… tetraethyl…, etc. In constructing the name, substituents are arranged in alphabetical order, without regard for multipliers.

3. For cycloalkenes, begin numbering at the double bond and proceed through the double bond in the direction to generate the lowest number at the first point of difference.

One of the most common mistakes in naming cycloalkenes is to generate the lowest number sequence around the ring, disregarding this rule. Once again, the numbering must begin at the double bond and proceed through the bond in the direction to generate the lowest number sequence.

4. Assign stereochemistry using the E-Z designation

Historically, alkenes have been named using cis- and trans- to represent stereochemistry around the double bond; cis- for compounds where the “main substituents” are on the same side of the double bond, and trans- when they are on opposite sides. This system clearly breaks down, however, in more complex molecules where decisions concerning the “main substituents” are not easily made, and the E-Z system provides a set of rules to aid in these decisions.

February 1, 2010 Posted by | Uncategorized | Leave a comment

Hydration of Alkenes

The addition of water (hydration) across the double bond of an alkene yields an alcohol. Alkenes are available as products of coal tar and petroleum refining,and a variety of catalytic conditions can support the addition of water across the double bond. In most cases, water adds in the direction that places the new hydroxyl group on the more highly substituted end of the double bond according to the Markovnikov rule, as in acid-catalyzed hydrations. (The more highly substituted end of the double bond is the one that is bonded to more carbon atoms.

February 1, 2010 Posted by | Hydration Of Alkenes | Leave a comment