PRPM112 LAB: EX 8

• ALCOHOLS

  • organic compounds composed of an aliphatic carbon atom with a hydroxyl (-OH) functional group
  • very common and may exist freely in nature
  • exhibit some unique set of physicochemical properties mainly due to the presence of the hydroxyl functional group
•  the hydroxyl group is a functional group of all alcohols, hence the general formula of alcohol


  ROH, where R is the alkyl group
• some of the properties of alcohols depend on the

  length of the alkyl chain attached to the hydroxyl group
• primary (1°) alcohol

  • C with the –OH group attached to one other carbon atom
  • general formula: RCH2OH
• secondary (2°) alcohol

  • C with the –OH group attached to two other carbon atoms
  • general formula: R2CHOH
• tertiary (3°) alcohol

  • C with the –OH group attached to three other carbon atoms
  • general formula: R3COH
• PROPERTIES OF ALCOHOLS

  BOILING POINT

  • generally have higher boiling points as compared to hydrocarbons having the same molecular masses
  • mainly due to the intermolecular H-bonding in between the OH groups

  
  

  1. boiling point increase = number of C in the aliphatic chain increase
  2. boiling point decrease = branching in aliphatic carbon chain increase
• PROPERTIES OF ALCOHOLS

  SOLUBILITY

  • governed by its ability to perform intermolecular H-bonding with polar solvents
  • H-bonds are usually formed in between the OH group of alcohol and water (which makes alcohol soluble with H2O)
  • however, the longer the alkyl chain is, the lesser the alcohol becomes soluble with water
• PROPERTIES OF ALCOHOLS

  ACIDITY


  • acidic in nature because of their ability to react with active metals (e.g. sodium, potassium,) forming the corresponding alkoxide
  • acidity is due to the polarity of the hydroxyl bond

  
  

  1. acidity decrease = when an electron donating group is attached to the hydroxyl group as the electron density increases on the oxygen atom
• primary alcohols are more acidic than secondary and tertiary alcohols
• alcohols exhibit a range of spontaneous chemical reactions due to the cleavage of C-O and O-H bonds
• JONES TEST (CHROMIC ACID TEST)

  • n-butyl alcohol

  1. blue-green solution
  2. changes color much faster

  • sec-butyl alcohol

  1. blue-green solution
  2. changes color much slower

  • tert-butyl alcohol

  1. orange solution
  2. n/a
• JONES TEST (CHROMIC ACID TEST)


  • function of test: used to distinguish primary and secondary alcohols from tertiary alcohols
  • reagent: Jones reagent (chromium oxide CrO3 in sulfuric acid H2SO4 );
  • named after Ewart Jones
  • positive result: blue-green solution
• JONES TEST (CHROMIC ACID TEST)

  : detect polar functional groups that can be oxidized:

  • 1° alcohols and aldehydes are oxidized to carboxylic acid
  • 2° alcohols to ketones;

  the orange hexavalent chromium (Cr6+) is reduced to a blue-green trivalent chromium (Cr3+) species, which often precipitates in acetone
• LUCAS TEST

  • n-butyl alcohol

  1. clear colorless solution
  2. n/a

  • sec-butyl alcohol

  1. two-layer solution
  2. forms layer much slower

  • tert-butyl alcohol

  1. turbid solution
  2. immediate

  • benzyl alcohol

  1. turbid solution
  2. often immediate
• LUCAS TEST


  • function of test: used to distinguish tertiary and secondary alcohols from primary alcohols
  • reagent: Lucas reagent (zinc chloride ZnCl2 in concentrated hydrochloric acid HCl);
  • named after Howard Lucas
  • positive result: turbid solution/formation of two layers
• LUCAS TEST

  principle: SN1 mechanism to produce insoluble alkyl chloride which appears as white precipitate or cloudiness of the solution; benzyl, allylic and propargylic alcohols often give immediate results like tertiary alcohol
• ESTERIFICATION REACTION


  methanol + salicylic acid

  1. minty odor
  2. immediate
• ESTERIFICATION REACTION


  • function of test: used to demonstrate reactivity of alcohols with carboxylic acids when heated with acid catalyst
  • positive result: methanol with salicylic acid results to minty odor (methyl salicylate); esters results to characteristic fruity-like odor
• : Fischer esterification reaction; reaction is both slow and reversible
• TEST FOR METHANOL


  red-violet ring at the junction of the two layers with dark red flocculent precipitate
• TEST FOR METHANOL


  function of test: demonstrate oxidation of alcohols through partial oxidation and dehydrogenation using hot catalyst (heated copper)
• : methanol is oxidized to methanal (formaldehyde) using heated copper as catalyst; to detect if methanol was oxidized, resorcinol is added to the solution and a concentrated sulfuric acid) is added dropwise to induce color change, precipitates and layer formation (method was introduced by Robert Mulliken)
• IODOFORM TEST

  • ethanol

  1. clear colorless solution

  • isopropyl alcohol

  1. turbid yellow solution with canary yellow precipitate

  • methanol

  1. clear colorless solution
• IODOFORM TEST


  • function of test: used to identify methyl ketones or secondary alcohols adjacent to a methyl group
  • reagent: Wagner’s reagent (iodine in potassium iodide); sodium hydroxide is used to decolorize the solution
  • positive result: turbid yellow solution/canary yellow precipitate
• I2 in KI in NaOH reacts only for compounds with the structure RCH(OH)CH3 or RC=OCH3 to form iodoform (canary yellow precipitate); does not work for all alcohols or ketones and does not work also for water-insoluble compounds
• ACROLEIN TEST

  glycerol

  • pungent acrid odor
• ACROLEIN TEST


  function of test: used to detect the presence of glycerol

  positive result: pungent acrid odor
• glycerol 


  polyhydroxy alcohol (triol) obtained as a by-product from the manufacture of soap used as a moisturizing agent in cosmetic formulations
• when heated with potassium bisulfate (KHSO4 ), glycerol is dehydrated to form propenal (acrolein)