{"id":1080,"date":"2019-10-31T14:09:03","date_gmt":"2019-10-31T14:09:03","guid":{"rendered":"http:\/\/chemdictionary.org\/?p=1080"},"modified":"2019-10-31T14:09:15","modified_gmt":"2019-10-31T14:09:15","slug":"dehydration-of-alcohols","status":"publish","type":"post","link":"https:\/\/chemdictionary.org\/dehydration-of-alcohols\/","title":{"rendered":"Dehydration of Alcohols"},"content":{"rendered":"\n<h2>Introduction<\/h2>\n\n\n\n<p>When alcohol is\nallowed to react with protic acids, it is prone to lose a water molecule to\nform alkenes. This type of reaction is commonly known as dehydration of\nalcohols. This is a basic example of an elimination reaction. The mechanism\nrate is different for primary, secondary and tertiary alcohols. In the case of\ntertiary, the carbonation is much stable so the rate of hydration is greatest\nfor tertiary alcohols as compared to primary and secondary alcohols. The\ndehydration process takes place in the three-step mechanism. <\/p>\n\n\n\n<h2>Mechanism of\ndehydration of alcohol<\/h2>\n\n\n\n<p>The dehydration\nof alcohol follows the E1 or E2 mechanism. The primary alcohols follow the E2\nmechanism for elimination reaction while the E1 mechanism is followed by\nsecondary and tertiary alcohols. Normally, it is a three-step mechanism. The\nsteps are explained as follows<\/p>\n\n\n\n<ol><li>The\nformation of protonated alcohol<\/li><li>The\nformation of carbonation<\/li><li>The\nformation of alkenes<\/li><\/ol>\n\n\n\n<ul><li>The\nformation of protonated alcohol<\/li><\/ul>\n\n\n\n<p>This\nstep involves the reaction of alcohol by a protic acid. Due to the presence of\na single lone pair on the oxygen atom, it acts as a Lewis base. The protonation\nof alcoholic oxygen makes it a good leaving group. This step is not complicated\nand hence quickly reversible.<\/p>\n\n\n\n<ul><li>The\nformation of a carbocation<\/li><\/ul>\n\n\n\n<p>In\nthis step, the breakdown of the C-O bond takes place which generates a\ncarbocation. This step is considered as the slowest step in the mechanism of\ndehydration of an alcohol. Hence, the formation of the carbocation is\nconsidered to be the rate-determining step. <\/p>\n\n\n\n<ul><li>The\nformation of alkenes<\/li><\/ul>\n\n\n\n<p>This is the ultimate step in the dehydration of alcohols. Here, in this step, the generated proton is eliminated with the help of a base. The carbon atom near to the carbocation breaks the present C-H bond to form C=C. therefore, an alkene is produced.<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"569\" height=\"328\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-104.png\" alt=\"\" class=\"wp-image-1081\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-104.png 569w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-104-300x173.png 300w\" sizes=\"(max-width: 569px) 100vw, 569px\" \/><\/figure><\/div>\n\n\n\n<p>Alcohol\ndehydration reaction<\/p>\n\n\n\n<p>A\ndehydration reaction is considered as that type of chemical reaction where\nwater is extracted from a single reactant. A production of alkene takes place\nwhen dehydration of an alcohol is carried out. A basic equation for alcohol\ndehydration is <\/p>\n\n\n\n<p>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C<sub>2<\/sub>H<sub>5<\/sub>OH&nbsp;&nbsp;&nbsp; &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; C<sub>2<\/sub>H<sub>4\n<\/sub>&nbsp;&nbsp;+&nbsp;&nbsp;&nbsp; H<sub>2<\/sub>O<\/p>\n\n\n\n<p>Alcohol dehydration is an example of an elimination reaction. An elimination reaction is the type of reaction in which two atoms adjacent to carbon atoms are eliminated from a molecule leaving multiple bonds between the carbon atoms.<\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" width=\"667\" height=\"138\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-105.png\" alt=\"\" class=\"wp-image-1082\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-105.png 667w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-105-300x62.png 300w\" sizes=\"(max-width: 667px) 100vw, 667px\" \/><\/figure>\n\n\n\n<p><br>\nDehydration of Primary, Secondary and Tertiary Alcohols<\/p>\n\n\n\n<p>Alcohols\nand ethers own leaving groups that are stronger Lewis bases than halide ion.\nthis property makes alcohols and ethers less reactive than the alkyl halides (where\nhalogen atoms replace one or more compounds of hydrogen atoms in an alkane).\nThey required to be protonated before undergoing a substitution or elimination\nreaction.<\/p>\n\n\n\n<p>The\nprimary, secondary and tertiary alcohol go through a process called the\nnucleophilic substitution reactions with HI, HBr and HCl to form alkyl halides.\nWith the help of chromic acid, the secondary alcohol gets oxidized to ketones\nand that of primary are oxidized to carboxylic acids.<\/p>\n\n\n\n<p>These\nare categorized as SN2 reactions in primary alcohols and SN1 reactions in\nsecondary as well as tertiary alcohols. Tertiary alcohols are easy to dehydrate\nbut on the other hand, primary alcohol dehydration is very tough.<\/p>\n\n\n\n<p>The\ndehydration of secondary and tertiary is known as an E1 reaction (it\u2019s a\ntwo-step mechanism), whereas the dehydration of primary alcohol is known as E2\nreaction (it\u2019s a one-step mechanism) because it is difficult to form primary\ncarbocations.<\/p>\n\n\n\n<p>Dehydration\nof alcohols using Aluminum Oxide as the catalyst<\/p>\n\n\n\n<p>The\ndehydration of ethanol to give Ethene<\/p>\n\n\n\n<p>It is considered the simplest way to make gaseous alkenes like ethene. if ethanol vapor is allowed to pass overheated aluminum oxide powder, the ethanol is cracked to generate ethene and water vapor. <\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" width=\"490\" height=\"69\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-106.png\" alt=\"\" class=\"wp-image-1083\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-106.png 490w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-106-300x42.png 300w\" sizes=\"(max-width: 490px) 100vw, 490px\" \/><\/figure>\n\n\n\n<p>To prepare some test tubes of ethene, the following <\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" width=\"491\" height=\"215\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-107.png\" alt=\"\" class=\"wp-image-1084\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-107.png 491w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-107-300x131.png 300w\" sizes=\"(max-width: 491px) 100vw, 491px\" \/><\/figure>\n\n\n\n<p>This method is\neasy to perform as just by boiling some ethanol in a flask and pass these vapor\nover aluminum oxide heated in a long tube.<\/p>\n\n\n\n<p>Dehydration of\nalcohols using an acid catalyst<\/p>\n\n\n\n<p>Concentrated\nSulphuric acid or concentrated phosphoric acid are normally used acid\ncatalysts.<\/p>\n\n\n\n<p>The results\nobtained from concentrated Sulphuric acid are messy. It is not only acid but\nalso a strong oxidizing agent. Some of the alcohol is oxidized to carbon\ndioxide by concentrated Sulphuric acid and simultaneously it reduces itself to\nSulphur dioxide. Both of these gases need to be removed from the alkene. It\nalso reacts with alcohol to generate a carbon mass.<\/p>\n\n\n\n<p>The dehydration\nof ethanol resulting ethene<\/p>\n\n\n\n<p>In an excess\npresence of concentrated Sulphuric acid ethanol is heated at a temperature of\n170 \u1d52C. the gases which are produced during the reaction are allowed to pass\nthrough sodium hydroxide solution to remove carbon dioxide and Sulphur dioxide\nreleased from side reactions.<\/p>\n\n\n\n<p>The resulted ethene is collected over water in the presence of conc. Sulphuric acid as given in a reaction below<\/p>\n\n\n\n<p><\/p>\n\n\n\n<figure class=\"wp-block-image\"><img loading=\"lazy\" width=\"467\" height=\"72\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-108.png\" alt=\"\" class=\"wp-image-1085\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-108.png 467w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-108-300x46.png 300w\" sizes=\"(max-width: 467px) 100vw, 467px\" \/><\/figure>\n\n\n\n<p>The Dehydration\nof cyclohexanol resulting cyclohexene<\/p>\n\n\n\n<p>This\npreparation is used to form and purify a liquid product. It is a fact that\ncarbon atoms that are joined in a ring make no difference to the chemistry of a\nreaction.<\/p>\n\n\n\n<p>Cyclohexanol is\nallowed to heat with concentrated phosphoric acid and the liquid cyclohexene is\ndistills off which later can be collected and purified.<\/p>\n\n\n\n<p>Here phosphoric acid is used instead of Sulphuric acid because it is much safe and produces a less messy reaction. Phosphoric acid is not a strong oxidizing agent.<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"457\" height=\"180\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-109.png\" alt=\"\" class=\"wp-image-1086\" srcset=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-109.png 457w, https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-109-300x118.png 300w\" sizes=\"(max-width: 457px) 100vw, 457px\" \/><\/figure><\/div>\n\n\n\n<p>The Dehydration\nof Complicated Alcohols<\/p>\n\n\n\n<p>There is a\npossibility of forming more than one alkene. Butan-2-ol is its good example\nwhich forms three different alkenes being formed when it is dehydrated.<\/p>\n\n\n\n<p>When alcohol is dehydrated, -OH group and a hydrogen atom from the next carbon atom in the chain is removed. There are two possibilities of happening with molecules like butan-2-ol. <\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"218\" height=\"162\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-110.png\" alt=\"\" class=\"wp-image-1087\"\/><\/figure><\/div>\n\n\n\n<p>This gives us these products:<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"247\" height=\"110\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-111.png\" alt=\"\" class=\"wp-image-1088\"\/><\/figure><\/div>\n\n\n\n<p>The\nproducts are but-1-ene, CH<sub>2<\/sub>=CHCH<sub>2<\/sub>CH<sub>3<\/sub> and\nbut-2-ene, CH<sub>3<\/sub>CH=CHCH<sub>3<\/sub>.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/p>\n\n\n\n<p>The situation is more complicated than it looks because but-2-ene displays geometric isomerism. Mixtures of two isomers are formed \u2013cis-but-2-ene and trans-but-2-ene.<\/p>\n\n\n\n<div class=\"wp-block-image\"><figure class=\"aligncenter\"><img loading=\"lazy\" width=\"295\" height=\"112\" src=\"https:\/\/chemdictionary.org\/wp-content\/uploads\/2019\/10\/image-112.png\" alt=\"\" class=\"wp-image-1089\"\/><\/figure><\/div>\n\n\n\n<p>The\n\u2013cis-but-2-ene is also termed as (Z)-but-2-ene while \u2013trans-but-2-ene is termed\nas (E)-but-2-ene.<\/p>\n\n\n\n<p>Result&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; <\/p>\n\n\n\n<p>Dehydration of\nbutan-2-ol results in a mixture containing<\/p>\n\n\n\n<ul><li>But-1-ene<\/li><li>Cis-but-2-ene\n(also (Z)-but-2-ene)<\/li><li>Trans-but-2-ene\n(also (E)-but-2-ene)<\/li><\/ul>\n\n\n\n<p>References<\/p>\n\n\n\n<p>https:\/\/www.chemguide.co.uk\/organicprops\/alcohols\/dehydration.html<\/p>\n\n\n\n<p>https:\/\/byjus.com\/chemistry\/dehydration-of-alcohols\/<\/p>\n\n\n\n<p>https:\/\/www.vedantu.com\/chemistry\/dehydration-of-alcohols<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Introduction When alcohol is allowed to react with protic acids, it is prone to lose a water molecule to form alkenes. This type of reaction is commonly known as dehydration of alcohols. This is a basic example of an elimination reaction. The mechanism rate is different for primary, secondary and tertiary alcohols. In the case &#8230; <a title=\"Dehydration of Alcohols\" class=\"read-more\" href=\"https:\/\/chemdictionary.org\/dehydration-of-alcohols\/\" aria-label=\"More on Dehydration of Alcohols\">Read more<\/a><\/p>\n","protected":false},"author":6,"featured_media":0,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"rating_form_position":"","rating_results_position":"","mr_structured_data_type":""},"categories":[8,7],"tags":[],"multi-rating":{"mr_rating_results":[{"adjusted_star_result":5,"star_result":5,"total_max_option_value":5,"adjusted_score_result":5,"score_result":5,"percentage_result":100,"adjusted_percentage_result":100,"count":2,"post_id":1080}]},"_links":{"self":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1080"}],"collection":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/comments?post=1080"}],"version-history":[{"count":2,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1080\/revisions"}],"predecessor-version":[{"id":1106,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/posts\/1080\/revisions\/1106"}],"wp:attachment":[{"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/media?parent=1080"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/categories?post=1080"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/chemdictionary.org\/wp-json\/wp\/v2\/tags?post=1080"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}