الرئيسية > alkanes, Pharmaceutical Organic Chemistry > ALKANES, part 2, Preparation of Alkanes

ALKANES, part 2, Preparation of Alkanes


Pharmaceutical Organic Chemistry, Saturated Hydrocarbons

ALKANES, part 2, Preparation of Alkanes

Clinical Pharmacy, Pharmaceutical Organic Chemistry Section

A study in Pharmaceutical Organic Chemistry as a revision for professionals & students

Studied & Revised By Ismail Mortada

B.Sc.Pharmacy & Health Sciences, Clinical Pharmacy Section
Under the process of Masters in Business Adminstiration, Pharmaceutical Marketing

هذه المقالة تعنى باستكمال مراجعة ودراسة أول طائفة من طوائف المواد الكيميائية العضوية (الموجودة والمكونة للكائنات الحية) وهي من المواد الشيقة والمهمة جدا في التطبيقات الطبية, البيولوجية, الأحيائية والصيدلانية, وبشكل مخصص وبعد أن تحدثنا في مقالات سابقة عن ماهية وتعريف المركبات العضوية, وبدأنا بشرح أبسط أنواعها او أقلها تعقيدا مما يسمى بالألكانات, نعنى الآن بطريقة تحضير هذه المواد (الألكانات) كيميائيا وكيفية تركيبها….هي دراسة سريعة ولكن أساسية…ودمتم سالمين..أخوك إسماعيل مرتضى

In last lecture we spoke about saturated hydrocarbons, basicaly the alkanes & we spoke about them, their nomencalture & different properties….etc, Now in this section of this topic we’ll discuss more about the preparation of alkanes

And although most alkanes upto pentane are obtained normaly from petroleum by fractional distillation, but still more complex alkanes can be prepared

Alkanes preparation can be achieved within different models & mechanisms, all have their own characteristics & ways including th following

A) Catalytic Hydrogenation of Alkenes
B) Hydrolysis of a Grignard Reagent
C) The Wurtz Reaction, which is called Dimerization

A) Catalytic Hydrogenation of Alkenes

This is a kind of what is called Additional reactions depends on the following points in which the carbon-carbon double bond in alkenes can add a mole of hydrogen in the presence of a suitable catalyst to give an alkane, in other words, adding hydrogen atom to a double bonded alkene structure in the presence of a catalyst will give us an alkane

The general mechanism includes the dissolving of an alkene in a suitable solvent in the presence of the catalyst, then hydrogen gas is passed (bubbled) at low pressure in the reaction vessel with constant stirring

Catalysts used can be such as the following
A) Platinum Pt
B) Nickel Ni
C) Palladium Pd

Example 1) For example the conversion of Ethylene to Ethane as the following

CH2=CH2  + H2 —-> CH3-CH3 In the presence of Pt & Pressure

In which the following
CH2=CH2 is Ethylene
CH3-CHE is Ethane

Example 2) Another example is the formation of n-hexane from 1-Hexene

CH3-CH2-CH2-CH2-CH=CH2 + H2 —-> CH3-CH2-CH2-CH2-CH2-CH3 In the presence of Ni & Presuure

In which the following
CH3-CH2-CH2-CH2-CH=CH2 is 1-Hexene
CH3-CH2-CH2-CH2-CH2-CH3 is n-Hexane

B) Hydrolysis of a Grignard Reagent

A) Grignard reagent is an alkyl magnesium halide compound, R-Mg-X
B) The Grignard reagent is formed when a solution of an Alkyl Halide (R-X) is allowed to stand over a metallic magnesium in the presence of anhydrous diethyl ether, a strong reaction will occur, the solution will turn cloudy & start to boil, The magnesium metal disappears & the resulting alkyl magnesium halide is called Grignard Reagent, this reaction is so active in a way that special precautions must be taken while preparing it
C) Than the addition of water to the grignard reagent will form alkane

Precautions taken when preparing Grignard Reagent are as the following

A) There must be no water present in the reaction system
B) There must be no other functional groups available in which the Griganrd Reagent may react with, such as aldehydes, ketones, alcohols, amines & carboxylic acids, the presence of any of these groups destroys the reagent before it has the chance to produce the alkane or the needed product

The Grignard Reagents are widely used as an intermediates in organic synthesis of alcohols & carboxylic acids

The General Equation would be as the following

R-X + Mg —–> R-Mg-X ; this is in the presence of Dry Ether Step 1

R-Mg-X + H-OH —–> R-H + MG X OH ; Step 2

In which the following
R-X = Alkyl Halide
Mg= Magnesium
R-Mg-X= Alkyl Magnesium Halide or as called Grignard Reagent
H-OH= Water

EXAMPLE): The preparation of n-Butane through the hydrolysis of Grignard Reagent

CH3-CH2-CH(Br)-CH3 + Mg —–> CH3-CH2-CH(MgBr)-Ch3
sec-Butyl bromide + Mg ——> sec-butyl magnesium bromide or grignard reagent

CH3-CH2-CH(MgBr)-CH3 + H-OH —–> CH3-CH2-CH2-CH3 + MgBrOH

C) The Wurtz Reaction: Dimerization

A) This is the reaction of alkyl halides with metallic sodium to give symmetrical alkanes, dimer in which di=two & mer=part this resulted dimer is for the past mentioned Alkyl Halide
B) The wurtz reaction is a poor method for the preparation of unsymmetrical alkanes; R-R’ : Where R’ is not equal to R, Because the combination of 2 or more different Alkyl halides in the presence of sodium produces a mixture of products that are difficult to separate
C) Unsymmetrical higher-molecular-weight alkanes can be produced by the coupling of a lithium dialkyl copper (an organometallic compound of general formula R2CuLi) with an alkyl halide

The general equation would be as the following

R-X + R-X + 2 NA ——-> R-R + 2 NAX
Alkyl halide ——-> symmetrical alkane + salt

EXAMPLE) The production of n-Butane from ethyl chloride

CH3-CH2-Cl + CH3-CH2-CL + 2 Na ———> CH3-CH2-CH2-CH3 + 2 NaCl
2 ethyl chloride ——–> n-Butane

As seen from the past equation & example on it that the resulted alkane is a symmetrical form, because it is difficult to produce unsymmetrical forms from this kind of reactions, the following equation & example will show the other side of how to produce unsymmetrical alkanes from chemical reactions

The general equation would be as the following

R2-Cu-Li + R’-X ———> R-R’ + R-Cu + LiX
Lithium dialkyl copper + alkyl halide ——-> unsymmetrical alkane

EXAMPLE) The production of n-Pentane from n-Propyl bromide

CH3CH2-Cu(CH3CH2)-Li + CH3-CH2-CH2-Br ———-> CH3-CH2-CH2-CH2-CH3
This reaction librates -CH3CH2Cu & -LiBr
Lithium diethyl copper + n-propyl bromide ———-> n-Pentane

From the past equations & reactions it shows that the resulted alkane was unsymmetrical

Alkanes preparations are the simplest in the organic chemistry field & although they are not exactly simple but when we move on toward discussing more elements of organic chemistry you’ll discover that chemistry & especially the organic section is very deep & concerned with the most difficult deep details

Pharmaceutical Organic Chemistry, Saturated Hydrocarbons

ALKANES, part 2, Preparation of Alkanes

Clinical Pharmacy, Pharmaceutical Organic Chemistry Section

A study in Pharmaceutical Organic Chemistry as a revision for professionals & students

Studied & Revised By Ismail Mortada

B.Sc.Pharmacy & Health Sciences, Clinical Pharmacy Section
Under the process of Masters in Business Adminstiration, Pharmaceutical Marketing

About these ads

أضف تعليق

إملأ الحقول أدناه بالمعلومات المناسبة أو إضغط على إحدى الأيقونات لتسجيل الدخول:

WordPress.com Logo

You are commenting using your WordPress.com account. تسجيل الخروج / تغيير )

Twitter picture

You are commenting using your Twitter account. تسجيل الخروج / تغيير )

Facebook photo

You are commenting using your Facebook account. تسجيل الخروج / تغيير )

Google+ photo

You are commenting using your Google+ account. تسجيل الخروج / تغيير )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 32 other followers

%d bloggers like this: