Alcohols, amines and organometallic

• C-OH bond is polarised but OH is a poor leaving group because of the anion -OH
• Pre-protonation enhances the polarisation and converts it into a good -OH2+ leaving group
• With enhanced polarisation and better leaving group, a good Nu like Br- can react with alcohol, weaker Nu like Cl- cannot react so a lewis acid is used which enhances polarisation so all the alcohol is in the complexed reactive form
• Tertiary alcohols after pre-protonated will react via Sn1 and has a carbenium ion intermediate mechanism - if the resulting carbenium ion is stabilised, -OH2+ group will leave without needing to be pushed by Nu.

  Tertiary alcohol undergo Sn1 in acidic conditions and if a Nu is present
• -OH can be converted to a better leaving group like -O-P(=O)Cl2 which enhances the + at C
• The alcohol functional group is versatile and common in synthetic organic chemistry, as it can be substituted easily through modifications that are usually by a transformation into a better leaving group
• Alcohols undergo E1 under acidic conditions without a Nu. E1 is favoured by tertiary and secondary alcohols as primary alcohols require high temperatures
• Basicity is quantified by the acidity of the conjugate acid
• large Ka and low negative Ka = strong conjugate acid = weak base
• R-OH can be a base by using its lp to donate to H+, expulsion of H+ regains the lp
• Alcohol can be used as an acid by O-H bond dissociating to R-O- and H+, so RO-H is a proton donor
• Acidity is quantified by pKa
• Alcohols are very weak acids so equilibrium lies to the left
• Alcohol is a very weak base and only protonated in strong acid like H2SO4
• Small Ka and high positive pKa = weak acid = strong conjugate base
• In UV-vis and MS, simple alcohols have no prominent characteristic features
• H-bonding in amines makes them relatively high-boiling and polar
• Amines are seen as moderate bases as lp can be used to bond reversible to H+
• An amine can act as an acid but equilibrium is well to the left so only a strong base will allow a conjugate base (amide) to be formed
• LDA (lithium diisopropylamide) is a strong base much used in organic chemistry
• lp can be regained by loss of H+ by using a base stronger than amine like hydroxide - lp in the product means that the more substituted amine is also a Nu so may react further until no H is left on the N
• Hofmann elimination favours the less substituted alkene
• In UV-vis, amine functional group will show no characteristic uv-visible absorbance
• Organometallic compounds are:

  • often very reactive
  • air and/or moisture sensitive
  • rarely isolated
  • mostly prepared in situ in solvent
• Type of bonding (ionic/polarised/covalent) in organometallic compounds depends on electronegativity of the metal relative to C
• Chemistry of Na, K, Li and Mg organometallic is dominated by carbanion-like behaviour
• Chemistry of Pb, Sn, Hg and Ti organometallic is dominated by radical-like behaviour
• Most organometallic compounds are prepared by reaction of metal with a halide
• Organometallics from the more electropositive metals are strong bases and can deprotonate almost any -H species except non-activated C-H as these react violently with the relatively acidic water/alcohol and are destroyed
• Transition metals can form organometallic intermediates and coordinate alkenes allowing useful coupling reactions