Join TSR
 
 About Us | FAQs | Sign in
 
HomeForumsArticlesRevision NotesPersonal StatementsUniversity GuidesHealth & RelationshipsPostgraduate StudyCareersFreshers
Join the UK's largest student community  
Advanced
Search

Join The Student Room Today

Be part of the UK's largest and fastest growing student community.

It's free to join and a lot of fun - Get inspired, express your ideas, interact and share

Join The Student Room Today

Revision:Chains and Rings - Organic Chemistry

From The Student Room

TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Chains and Rings - Organic Chemistry

Contents

Aldehydes and Ketones

Reduction

Reagents: NaBH4

Reaction: Nucleophilic Addition


Aldehyes are reduced to primary alcohols

\displaystyle \mathsf{CH_3CH_2CHO + 2[H]} \longrightarrow \mathsf{CH_3CH_2CH_2OH}


Ketones are reduced to secondary alcohols

\displaystyle \mathsf{CH_3COCH_3 + 2[H]} \longrightarrow \mathsf{CH_3CH(OH)CH_3}


Aldehyde or Ketone???

  • Reaction with 2,4-dinitrophenylhydrazine
  • Add this to mixture, get bright orange precipitate if aldehyde or ketone present.

THEN,

  • Using the product from this reaction
    • recystallise
    • Measure Melting Point
    • Use data book to find out structure


Test for aldehydes

Oxidation tests because only aldehydes undergo oxidation! RCH0 + [0] gives RCOOH.

1st method

  • refluxing with acidified dichromate(VI)(aq) ions
  • Results: Orange to green shows aldehyde has been oxidised to carboxylic acid.

2nd method

  • use tollens reagent (made by ammonia & silver nitrate) warm this mixture.
  • Results: Silver mirror forms if aldehyde has been oxidised to a carboxylic acid.
  • Aldehyde gets oxidised to carboxylic acid, while Ag+ ions are reduced to silver metal.


Another reaction we must know:

Formation of 2-hydroxypropanoic acid using ethanal

  • Stage 1
    • Reagents: HCN
    • Conditions KCN
    • Reaction Nucleophilic Addition
    • Equation: \displaystyle \mathsf{CH_3CHO + HCN} \longrightarrow \mathsf{CH_3CH(OH)CN}
  • Stage 2
    • Reagents: H+ (aq)
    • Conditions : Warm, Reflux
    • Reaction Hydrolysis
    • Equation: \displaystyle \mathsf{CH_3CH(OH)CN + 2H_20 + H^+} \longrightarrow \mathsf{CH_3CH(OH)COOH + NH_4}


NMR Checklist

  • Count the number of peaks
  • Explain the positions of the peak:
  • Say what type of proton (e.g. the protons in the CH3 group)
  • Say where the chemical shift is
  • Say ‘this is consistent with the data sheet value’
  • How many protons are in that environment
  • Spitting (use n + 1 ) rule, where n is the number of hydrogen’s on the adjacent carbon


Use of D2O in N.M.R.

  • Protons in -OH are labile.
  • If an organic molecule contains –OH groups & is mixed with deuterium oxide then the protons are replaced with deuterium atoms . Since the deuterium atom has an even number of particles in its nucleus (a proton and a neutron) it does not show up in proton N.M.R.
  • So, if spectra are taken of a molecule before and after the use of D2O, a comparison of the two spectra can reveal any labile hydrogens in the molecule.


Arenes

Structure of Bezene

  • Planar Molecule
  • All C-C bonds same length
  • P orbitals above and below the ring
  • Overlapping of p-orbitals forms pi bonds
  • Electrons are delocalised


Resistance to electrophilic addition

  • Doesn't polarise electrophiles well
  • Electrophiles less attracted
  • Stable delocalised system would need to be disrupted


Electrophilic Substitution

  • Nitration of bezene
  • Reagents: \displaystyle \mathsf{HNO_3 and H_2SO_4} (catalyst)
  • Conditions: approx. 60°C
  • Equation: \displaystyle \mathsf{C_6H_6 + HNO_3} \longrightarrow \mathsf{C_6H_5NO_2 + H_20}
  • Mechanism:
    • Step 1 = generation of electrophile: \displaystyle \mathsf{2H_2SO_4 + HNO_3 2HSO_4^- + NO_2+ + H_3O^+}
    • Step 2 = Electrophilic Substitution
    • Step 3 = regeneration of catalyst: \displaystyle \mathsf{H^+ + HS0_4^-}\longrightarrow \mathsf{H_2SO_4}


  • Halogenation (can do this with Br2 as well)
  • Reagents : Cl2 and ALCl3 (catalyst)
  • Conditions: anhydrous
  • Equation: \displaystyle \mathsf{C_6H_6 + Cl_2} \longrightarrow \mathsf{C_6H_5Cl + HCl}
  • Mechanism:
    • Step 1 = generation of electrophile: \displaystyle \mathsf{Cl_2 + AlCl_3}\longrightarrow \mathsf{ Cl^+ + AlCl_4}
    • Step 2 = Electrophilic Substitution
    • Step 3 = regeneration of electrophile: \displaystyle \mathsf{H^+ + AlCl_4} \longrightarrow \mathsf{AlCl_3 + HCl}


Phenols

Form salts by its reactions with NaOH and Na

\displaystyle \mathsf{C_6H_5OH + NaOH} \longrightarrow \mathsf{C_6H_5O-Na^+ + H_20}

\displaystyle \mathsf{C_6H_5OH + Na} \longrightarrow \mathsf{C_6H_5O-Na^+ + \frac{1}{2} H_2}

Phenols react with bromine. the bromine is decolorised and white cystals of 2,4,6-tribromophenol are formed.


Phenol Vs Bezene

  • Phenol does not require halogen carrier & reacts instantly with bromine
  • OH- group activates benzene ring
  • This increases electron density around ring (especially at 2,4, 6)
  • This polarises the halogen & increases the attraction for the halogen


Uses of phenols

TCP is used as antiseptics & disinfectants!


Polymers

Addition Polymerisation

  • Sigma bond breaks
  • Many molecules join on


Addition Polymers can have different structures because:

  • For any polymer other than poly(ethene), the carbon atom attached to the R group in the alkene unit becomes a chiral centre when a polymer chain is formed: (with 4 different groups attached)


Side groups can be arranged in either:

  • Alternating chirality = Syndiotactic
  • Random chirality = Atactic
  • Same chirality = Isotactic


Properties

Isotactic = chains closely packed = Strong intermolecular forces = High Melting Point

Syndiotactic = chains closely packed = Strong intermolecular forces = High Melting point

Atactic = lack of regularity makes it impossible for the chains to lie closely together = Weaker intermolecular forces = Low Melting point


Condensation Polymerisation

  • A molecule of water is eliminated
  • Ester or peptide link will be formed
  • Polyamides = di-carboxylic acid + diamine = amide link
  • Polyesters = di-carboxylic acid + diol = ester link
  • Polypeptides/Proteins = many amino acids joined = amide link


Polymers we MUST know (for OCR)

  1. Terylene - Monomers = benzene-1,4-dicarboxylic acid and ethane-1,2-diol
    • Uses = fibres, clothing
  2. Kelvar - Monomers = benzene-1,4-dicarboxylic acid and benzene-1,4-diamine
    • Uses = bullet proof clothing, tennis rackets
  3. Nylon-6,6 - Monomers = hexane-1,6-dicarboxlic acid and 1,6-diaminohexane
    • Uses = fibres, clothing


Dipeptides

  • Two amino acids join together.
  • Via condensation reaction
  • COOH reacts with NH2 group.
  • Amide/Peptide link formed.
  • Can be two different structures because you can switch the two R groups around in position.


Polypeptides

Many amino acid monomers join in condensation reaction, forming amide links


Primary Amines

General formula = RNH2


Reactions

With water:

\displaystyle \mathsf{CH_3CH_2NH_2 + H_20} \longrightarrow \mathsf{CH_3CH_2NH_3^+ + OH^-}


With acids:

\displaystyle \mathsf{NH_3 + HCl} \longrightarrow \mathsf{NH_4Cl^-}

\displaystyle \mathsf{CH_3CH_2NH_2 + HCl} \longrightarrow \mathsf{CH_3CH_2NH_3 + Cl^-}


How primary amines act as bases

  • Accepts a proton
  • To give NH3+
  • Using LP of electrons on N of amine group
  • Dative covalent bond formed between N and H


Basicity

  • Depends on the electron density of the lone pair of electrons on the nitrogen atom.
  • Greater e- density = greater base strength


Alkyl groups

  • Have positive inductive effect
  • Push e- away towards the nitrogen atom
  • Increase electron density on the nitrogen atom


Aromatic Amines

  • Have negative inductive effect
  • Pull electrons away the nitrogen atom
  • A benzene ring would allow the lone pair of e- to be delocalised into the ring
  • Decreases the electron density on the nitrogen atom
  • Less likely to capture a proton


Basicity Order

  • Aromatic groups = weakest
  • Ammonia = middle
  • Alkyl groups = strongest




Comments

Retrieved from "http://thestudentroom.co.uk/wiki/Revision:Chains_and_Rings_-_Organic_Chemistry"
Article Tools:  Create New Article  |  Report This Article  |  This Article's History  |