Preparation of Alkanes PDF

Preparation of alkanes Organic chemistry Preparation of alkanes 1. Catalytic Hydrogenation of Alkenes The double bond of an alkenes consists of a sigma (σ) bond and a pi (π) bond. Because the carbon-carbon (π) bond is relatively weak, it is quite reactive and can be easily broken and reagents can be...

Preparation of alkanes Organic chemistry Preparation of alkanes 1. Catalytic Hydrogenation of Alkenes The double bond of an alkenes consists of a sigma (σ) bond and a pi (π) bond. Because the carbon-carbon (π) bond is relatively weak, it is quite reactive and can be easily broken and reagents can be added to carbon. Preparation of alkanes The most important of these methods is the hydrogenation of alkenes. When shaken under a slight pressure of hydrogen gas in the presence of a small amount of catalyst, alkenes are converted smoothly and quantitatively into alkanes of the same carbon skeleton. Catalyst, in chemistry, any substance that increases the rate of a reaction without itself being consumed. Preparation of alkanes 2. Reduction of alkyl halides (R-X) Alkyl halide (Compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms (fluorine, chlorine, bromine or iodine)). a) hydrolysis of Grignard reagent A Grignard reagent or Grignard compound is a chemical compound with the generic formula R-Mg-X where X is a halogen (The halogen atoms are fluorine (F), chlorine (Cl), bromine (Br) and iodine (I)) and R is an organic group, normally an alkyl. Preparation of alkanes Preparation of alkanes b) Reduction by metal and acid RX + Zn + H+ RH + Zn++ + X- Preparation of alkanes Reduction of an alkyl halide, either via the Grignard reagent or directly with metal and acid involves simply the replacement of a halogen atom; by a hydrogen atom the carbon skeleton remains intact. This method has about the same applicability as the previous method, the hydrogenation of alkenes would probably be preferred because of its simplicity and higher yield. Preparation of alkanes The Grignard reagent: an organometallic compound When a solution of an alkyl halide in dry ethyl ether, (C2H5)2O, is allowed to stand over turnings of metallic magnesium, a vigorous reaction takes place. The resulting solution is known as a Grignard reagent, It is one of the most useful and versatile reagents known to the organic chemist. Preparation of alkanes The Grignard reagent has the general formula RMgX and the general name alkyl magnesium halide. The magnesium becomes bonded to the same carbon that previously held halogen. E.g. (n-propyl chloride yields n propyl magnesium chloride) e.g. (isopropyl chloride yields isopropyl magnesium chloride) Preparation of alkanes The Grignard reagent is the best known member of a broad class of substances, called organometallic compounds, in which carbon is bonded to a metal: lithium, potassium, sodium, zinc, mercury, lead, thallium almost any metal known. The carbon-metal bond like the one in the Grignard reagent is highly polar ((Polar compounds are those with distinct regions of positive and negative charge)). Therefore, organometallic reagents are strong nucleophiles Preparation of alkanes Using the Grignard reagent as an example, the carbon is more electronegative than magnesium, so the bonding pair of electrons is pulled towards the carbon creating a partial negative charge. Grignard reagents are strong nucleophiles Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Electrons in the bond are pulled towards the more electronegative carbon atom CH3CH2.. δ− The carbon atom becomes slightly negatively charged MgBr δ+ Preparation of alkanes 3- Coupling of alkyl halides with organometallic compounds The coupling of alkyl halides with organometallic compounds is the only one of these methods in which carbon-carbon bonds are formed and a new, bigger carbon skeleton is generated. Gilman Reagents Gilman Reagents Organolithium reagents react with cuprous iodide to give a lithium diorganocopper reagent (also called diorganocuprates), which often is referred to as a Gilman reagent. Remember that organolithium reagents are formed by a reaction of lithium metal with an alkylhalide Gilman Reagents Coupling of alkyl halides with organometallic compounds: To make an alkane of higher carbon number than the starting material requires formation of carbon-carbon bonds, most directly by the coupling together of two alkyl groups. Coupling takes place in the reaction between a lithium dialkyl copper, R2CuLi, and an alkyl halide, R’X. (R’ stands for an alkyl group that may be the same as, or different from, R.) Gilman Reagents An alkyllithium, RLi, is prepared from an alkyl halide, RX, in much the same way as a Grignard reagent. To it is added cuprous halide, CuX, and then, finally the second alkyl halide, R'X. Ultimately, the alkane is synthesized from the two alkyl halides, RX and R'X. Gilman Reagents For good yields, R'X should be a primary halide; the alkyl group R in the organometallic may be primary, secondary, or tertiary For example Mechanism Although the mechanism is not understood, evidence strongly suggests this much: the alkyl group R is transferred from copper, taking a pair of electrons with it, and attaches itself to the alkyl group R' by pushing out halide ion (nucleophilic aliphatic substitution). Examples: