Alcohols will burn, but this destructive oxidation is of no use...unless you happen to want to enjoy the energy stored in the bonds to cook on.
Oxidation is best described as the addition of oxygen or the removal of hydrogen [as water]. These statements describe exactly what happens to the molecules.
Oxidation that preserves the carbon skeleton is effected using potassium dichromate (VI) in dilute sulphuric acid. This is a delightful orange colour, and if it does its job it will end up green.
Alcohols are classified as primary, secondary or tertiary. This is determined by looking at the alcoholic carbon atom and counting how many other carbon atoms this is bonded to...and making the connection! That's fine except for methanol..but this is always classified as primary.
Oxidation of primary alcohols yields firstly the aldehyde and then subsequently the carboxylic acid. To stop at the aldehyde, add the oxidant to the warm alcohol and distill out the aldehyde as quickly as it is formed. The acid is obtained by arranging to heat the mixture under reflux.
Secondary alcohols will give ketones. Tertiary alcohols are resistant since the alcoholic carbon atom does not carry the hydrogen atom necessary for the first step of the reaction.
]]>The best definition therefore involves the concept of "oxidation number"..which can be defined as: "The formal charge that an atom would have if the bonding in the chemical species were considered to be wholly ionic". By the way, this is a definition that the examiner expects you to know. So learn it. In an oxidation reaction the oxidation number of an atom will increase...become more positive...as the atom loses units of negative charge [electrons].
Oxidation CANNOT happen in isolation...and its partner REDUCTION must occur simultaneously. Reduction will therefore absorb the electrons that the oxidation liberates. As a result the oxidation number will become smaller [more negative or less positive] as the atoms acquires units of negative charge [electrons]
In a REDOX recation then all of the electrons produced in the oxidation MUST be consumed in the accompanying reduction. There can be NO spare electrons!
]]>Enjoy!
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]]>The most polar may well be the C-Cl bond, but the most reactive is the C-I, since the latter is the longest and therefore the weakest. This fact is easily proven by the rate of the reaction with hydroxide ion, releasing the halide ion to react with aqueous silver (I) ion and give a ppt of silver iodide.
Remember that the haloalkanes may be prepared by reaction of alkenes with halogen or hydrogen halide [electrophilic addition] or from alkanes via a free radical substitution type of reaction.
]]>The reagent is concentrated sulphuric acid, but the conditions become milder as the classification moves from primary to secondary to tertiary. For example, ethanol is dehydrated with 95% sulphuric acid at 170 deg. to give ethene; butan-2-ol with 60% sulphuric acid at 100 deg and methylpropan-2-ol with 20% sulphuric acid at 85-90 deg.
Remember that in each case the product will be an alkene, but with some alcohols different isomeric alkenes are possible. For simplicity's sake consider the -OH group being removed together with a hydrogen atom from a neighbouring carbon atom, to make water. This can give the unsaturation either side of the original alcohol group, with geometric isomerism also a possibility, depending on the structure of the original alcohol. Do watch out for this!
]]>cis-3,4-dimethyldec-3-ene
4-chloro-nonan-1-ol
hexan-3-one
octanoic acid
3-bromopentanal
3-hydroxy-2-methylheptanal
Alkenes can be simply classified into symmetrical or non symmetrical. To differentiate these two you need to look at the "alkenic" carbon atoms..and check which atoms they are bonded to. If the are bonded to CC and HH; or CH and CC then the alkene is non symmetrical. If the alkenic atoms are bonded to CC and CC; or CH and CH; or HH and HH [ie ethene] then the alkene is symmetrical.
When then considering the orientation of addition of hydrgoen halides, remember the statement: " unto those that have shall more be given"..
in other words the hydrogen atom of the hydrogen halide will end up being bonded to the alkenic carbon atom that already has most hydrogen atoms.
Nucleophiles are electron pair donors, which use that pair to form a new bond to the substrate. If you are into A2 chemistry and interested in transition metal complex ions [and let's face it they ARE pretty amazing!] then nucleophiles could be called LIGANDS for these function in the same way!
Electophiles are electron pair acceptors; they accept a pair of electrons FROM the substrate to form a new bond.
Free radicals are very reactive intermediates which carry a single unpaired electron.
Alkanes are boring. Their olde name of "paraffins" was much more apt since it is derived from some Latin: "parum affinis" meaning 'little affinity'. See the benefits of a classical education? Since they contain only C-C and C-H bonds that are scarcely polar, electrophiles and nucleophiles are denied access, so we are left with free radicals. Since the substrate is Saturated, only Substitution can take place. Remember that free radicals [as their name implies] have no manners, and will give mixtures of products and multiple substitution with limited control. Their reactions with haloalkanes also give HX (g).
Alkenes contain a weak pi bond and, being UNsaturated, will suffer ADDITION reactions. The two pi type electrons are readily accessible to electrophiles, hence electrophilic addition reactions are the norm for alkenes. Whatever else happens the double bond does NOT break, for if it did the molecule would fall into two pieces!
Having put a halogen atom into the molecule we have now activated the substrate in a different way. The saturated haloalkanes now contain a polar C-Hal bond that will be subject to nucleophilic attack on the positively charged Carbon atom. Their reactions will therefore typically be nucleophilic substitution. Remember that the length and therefore strength of the C-Hal bond dictates reactivity NOT polarity, so the most reactive is C-I although the most polar is C-Cl.
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