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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirty nine (Ethanol)

Lesson thirty nine
ProfVietanhNgày: Thứ ba, 2010-06-01, 11:58 AM | Tin nhắn # 1
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Ethanol

Ethanol, also known as ethyl alcohol, drinking alcohol or grain alcohol, is a flammable, colorless, slightly toxic chemical compound, and is best known as the alcohol found in alcoholic beverages. In common usage, it is often referred to simply as alcohol. Its molecular formula is variously represented as EtOH, CH3CH2OH, C2H5OH or as its empirical formula C2H6O.


Chemical formula of ethanol

Ethanol has been used by humans since prehistory as the intoxicating ingredient in alcoholic beverages. Dried residues on 9000-year-old pottery found in northern mainland China imply the use of alcoholic beverages even among Neolithic peoples. Ethanol was first prepared synthetically in 1826, through the independent efforts of Henry Hennel in Great Britain and S.G. Sérullas in France. Michael Faraday prepared ethanol by the acid-catalysed hydration of ethylene in 1828, in a process similar to that used for industrial ethanol synthesis today.

Physical properties
Ethanol's hydroxyl group is able to participate in hydrogen bonding. At the molecular level, liquid ethanol consists of hydrogen-bonded pairs of ethanol molecules (dimers); this phenomenon renders ethanol more viscous and less volatile than less polar organic compounds of similar molecular weight. Ethanol, like most short-chain alcohols, is flammable, colorless, has a strong odor, and is volatile. It is miscible with water and with most organic liquids, including nonpolar liquids such as aliphatic hydrocarbons.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ ba, 2010-06-01, 12:05 PM | Tin nhắn # 2
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Chemical properties

The chemistry of ethanol is largely that of its hydroxyl group. Ethanol is classified as a primary alcohol, meaning that the carbon to which its hydroxyl group is attached has at least two hydrogen atoms attached to it as well.

Acid-base chemistry
Ethanol's hydroxyl proton is very weakly acidic; it is an even weaker acid than water. Ethanol can be quantitatively converted to its conjugate base, the ethoxide ion (CH3CH2O), by reaction with an alkali metal such as sodium. This reaction evolves hydrogen gas:

2CH3CH2OH + 2Na → 2CH3CH2ONa + H2

Nucleophilic substitution
In aprotic solvents, ethanol reacts with hydrogen halides to produce ethyl halides such as ethyl chloride and ethyl bromide via nucleophilic substitution:

CH3CH2OH + HCl → CH3CH2Cl + H2O

Note that the above requires catalyst such as zinc chloride
CH3CH2OH + HBr → CH3CH2Br + H2O

Ethyl halides can also be produced by reacting ethanol by more specialized halogenating agents, such as thionyl chloride for preparing ethyl chloride, or phosphorus tribromide for preparing ethyl bromide.

Esterification
Under acid-catalysed conditions, ethanol reacts with carboxylic acids to produce ethyl esters and water:

RCOOH + HOCH2CH3 → RCOOCH2CH3 + H2O

The reverse reaction, hydrolysis of the resulting ester back to ethanol and the carboxylic acid, limits the extent of reaction, and high yields are unusual unless water can be removed from the reaction mixture as it is formed. Esterification can also be carried out using more a reactive derivative of the carboxylic acid, such as an acyl chloride or acid anhydride.
Ethanol can also form esters with inorganic acids. Diethyl sulfate and triethyl phosphate, prepared by reacting ethanol with sulfuric and phosphoric acid, respectively, are both useful ethylating agents in organic synthesis. Ethyl nitrite, prepared from the reaction of ethanol with sodium nitrite and sulfuric acid, was formerly a widely-used diuretic.

Dehydration
Strong acids, such as sulfuric acid, can catalyse ethanol's dehydration to form either diethyl ether or ethylene:

2CH3CH2OH → CH3CH2OCH2CH3 + H2O
CH3CH2OH → H2C=CH2 + H2O

Which product, diethyl ether or ethylene, predominates depends on the precise reaction conditions.

Oxidation
Ethanol can be oxidized to acetaldehyde, and further oxidized to acetic acid. In the human body, these oxidation reactions are catalysed by enzymes. In the laboratory, aqueous solutions of strong oxidizing agents, such as chromic acid or potassium permanganate, oxidize ethanol to acetic acid, and it is difficult to stop the reaction at acetaldehyde at high yield. Ethanol can be oxidized to acetaldehyde, without overoxidation to acetic acid, by reacting it with pyridinium chromic chloride.

Combustion
Combustion of ethanol forms carbon dioxide and water:

C2H5OH + 3O2 → 2CO2 + 3H2O


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ ba, 2010-06-01, 12:06 PM | Tin nhắn # 3
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Production

Ethanol is produced both as a petrochemical, through the hydration of ethylene, and biologically, by fermenting sugars with yeast. Which process is more economical is dependent upon the prevailing prices of petroleum and of grain feedstocks.

Ethylene hydration
Ethanol for use as industrial feedstock is most often made from petrochemical feedstocks, typically by the acid-catalyzed hydration of ethene, represented by the chemical equation

C2H4(g) + H2O(g) → CH3CH2OH(l)

The catalyst is most commonly phosphoric acid, adsorbed onto a porous support such as diatomaceous earth or charcoal; this catalyst was first used for large-scale ethanol production by the Shell Oil Company in 1947. Solid catalysts, mostly various metal oxides, have also been mentioned in the chemical literature. The product formed from the reaction is usually in a liquid state, due to the high pressure system it is created in. It forms a vapor when it leaves the reaction vessel, due to the high temperature (300 degrees Celsius) that is used for the reaction.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirty nine (Ethanol)
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