Properties
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what happens to boiling point as alkane chain length increase and why?
• boiling point increases • number of e- increases • points of contact increases • more LDFs between alkane molecules
what does branching do to the boiling point of alkanes?
• branching decreases boiling point • less points of contact/branches get in the way • fewer LDFs between alkane molecules
alkane solubility and why
• alkanes are insoluble • alkanes are non-polar, water is polar • alkanes form LDFs • water forms H bonds • LDFs are not strong enough to overcome the H bonds between water molecules so will not surround themselves with them
trend in boiling point of alkanes
boiling point increases when: - chain length increases - branching decreases
why are alkanes relatively unreactive with most reagents?
• σ bonds are strong • C-C bonds are non-polar • C and H electronegativity are very similar so the C-H bond is also non-polar
products of incomplete combustion?
• CO(g) +H2O(l) OR • C(s) + H2O(l)
stereoisomerism
molecules with the same structural formulae but a different arrangement of the atoms in space
what is required for molecule to show optical isomerism?
chiral atom must be present
what is required for a molecule to show E/Z isomerism?
• C=C bond • each carbon in the C=C bond must be attached to two different groups
what is required for molecule to show cis-trans isomerism?
• C=C bond • each carbon in the C=C bond must be attached to two different groups one of which must be H
how are E/Z isomers determined?
• the atoms attached to each carbon atom in a double bond are given a priority based upon their atomic number • if the high priority groups are on the same side then the molecule is the E isomer vice versa
what do you assign priority if the atoms attached to both carbons are the same?
go along the branch to find the first point of difference
trend in reactivity of haloalkanes
• reactivity increases down alkanes of group 7 • polarity decreases (difference in electronegativity between the C-X decreases) • lower bond enthalpy therefore faster rate of reaction (e.g. hydrolysis)
how would you measure rate of hydrolysis of primary alkanes?
• carry out a hydrolysis reaction using haloalkanes (of each halogen) with water or NaOH • mix and react in the presence of AgNO3 • hydrolysis produces halide ions which will react with Ag+ to form a ppt • record time taken for ppt to form
most and least reactive primary haloalkanes and why?
most: iodoalkanes (C-I has the lowest bond enthalpy) least: fluoroalkanes (C-F has the highest bond enthalpy)