Series editors' preface - Wood Tools

R'
|
R – C O
The presence of the carbonyl group gives a
permanent dipole with some potential for
hydrogen bonding to H-donors such as water.
The overall polarity of ketones and aldehydes
depends on which groups are attached to the
carbon, e.g. non-polar methyl groups will
reduce the overall polarity of the solvent.
Ketones are more common than aldehydes in
conservation. Polar oxygenated solvents such
as alcohols, ketones and aldehydes are effective
solvents for many natural and synthetic
resins but are relatively poor solvents for
greasy dirt. Some common examples of
ketones are acetone, methyl ethyl ketone and
diacetone alcohol.
CH3 |
CH3 – C O
Acetone (propanone according to the IUPAC
system) is a fast-evaporating solvent that
rapidly swells and dissolves natural resins.
CH3 |
CH3 – CH2 – C O
Methyl ethyl ketone (butanone) has been
used in furniture conservation but is not as
common as acetone. The extra methylene
group makes MEK slightly less polar than
acetone.
CH3 |
|
OH
O
||
a ketone (neither R can be H)
acetone
CH 3 – C – CH 2 – C – CH 3
methyl ethyl ketone
diacetone alcohol
Diacetone alcohol combines both alcohol
and ketone groups in a single solvent. The
larger size of the molecule and its capacity for
hydrogen bonding means that it evaporates
slowly and the combination of functional
groups makes it a good solvent for a wide
range of resins.
Ethers
Ethers have the general formula R⎯O⎯R or
R⎯O⎯R 1. The functional group is an oxygen
atom with two lone pairs of electrons, bonded
to two alkyl groups, which may be different
Principles of cleaning 511
to each other. The structure of ethers is similar
to that of water but although the oxygen atom
is electronegative, the alkyl groups are only
slightly positive, giving an overall weakly polar
effect. They do not form hydrogen bonds, are
very volatile, are immiscible or only slightly
miscible with water and are not very reactive.
Ethers have been used to clean some solventsensitive
surfaces because their fast evaporation
rate limits their solubility effect. They are
not usually used in conservation because they
are extremely flammable and have strong
narcotic properties. Cellosolve (C 2H 5OC 2H 4OH)
has been used in conservation. Its solvent
properties are dominated by the alcohol group
rather than the ether group. Less potentially
harmful solvents (e.g. methoxypropan-2-ol)
are usually substituted for it.
R – O – R'
Ether functional group
CH3 – CH2 – O – CH2 – CH2 – OH
Ethyl cellosolve
Esters
Esters have the general formula RCOOR 1. The
functional group is a carbon atom doublebonded
to one oxygen atom and singlebonded
to another, with alkyl groups at either
end that may be different from each other.
Although the lone pairs of electrons on both
oxygens make esters polar, they do not hydrogen-bond
themselves. Like aldehydes and
ketones, esters hydrogen-bond with water.
Esters are very good solvents for nitrocellulose
and some are good solvents for polyvinyl
acetate adhesives.
O
||
R' – C – O – R
Ester functional group
O
||
CH3 – C – O – CH2 – CH3 Ethyl acetate
Organic nitrogenous compounds
Amines Amines contain a nitrogen atom. They
have the general formula R⎯NH 2 (the NH 2