Topic 10: Organic Chemistry AS: Halogenoalkanes, Alcohols and Spectra

Compare reactant and product connectivity, identify bonds broken and formed, and classify the specified step. Conditions help distinguish competition: aqueous KOH favours substitution/hydrolysis, while ethanolic KOH with heat favours elimination.

Every curly arrow starts at an electron pair and ends where a new bond or lone pair forms. Atom count and total charge must be conserved through every step.

For C$^{\delta+}$–Br$^{\delta-}$, both C–Br electrons move to bromine. Br$^-$ is electron-rich and can act as a nucleophile; the electron-deficient carbon species can be attacked by a nucleophile and is electrophilic.

Bond polarity makes one heterolytic direction more plausible: the more electronegative atom takes the pair, stabilising negative charge, while the other centre becomes electron-deficient.

Draw the curly arrow from the actual lone pair (or the atom bearing it) to the $\delta+$ carbon. The nucleophile is attracted by charge but defined by pair donation.

Mark the relevant dipole, identify the electron-pair donor and acceptor, then start each curly arrow at the donor pair. Bond strength and reaction conditions still affect whether the predicted route is fast or favoured.

Choose the longest carbon chain, number it to give substituents the lowest set of locants, name F/Cl/Br/I as fluoro-, chloro-, bromo- and iodo-, alphabetise different prefixes, and use di-, tri- or tetra- for repeats.

A structural formula groups connected atoms; a displayed formula shows every atom and bond; a skeletal formula uses vertices/line ends for carbon and omits attached C–H bonds while showing halogen symbols.

Locate the carbon directly bonded to the halogen, ignore the halogen and any hydrogens, then count how many other carbon atoms are directly attached to that carbon.

For an unsymmetrical secondary halogenoalkane, elimination can remove H from either adjacent carbon and may give positional and E/Z alkene products. Aqueous conditions instead favour alcohol formation.

For R–X + OH$^-$, mark C$^{\delta+}$–X$^{\delta-}$. Draw a curly arrow from the O lone pair to the C–X carbon and another from the C–X bond to X. The one-step substitution gives ROH and X$^-$.

Use excess ammonia to favour the primary amine and reduce further substitution. Full curly arrows start at lone pairs or bonds and all dipoles, charges and leaving groups must be shown.

In aqueous silver nitrate/ethanol, the halogenoalkane hydrolyses and released X$^-$ forms AgX. Shorter time to the first comparable cloudiness or precipitate means faster hydrolysis.

Use equal halogenoalkane amounts, identical AgNO$_3$/ethanol volumes and concentrations, the same water-bath temperature, simultaneous mixing and one objective endpoint. Ethanol helps the organic reagent mix with aqueous solution.

Independent variable is halogenoalkane structure or halogen; dependent variable is precipitation time. Control temperature, reagent concentrations/volumes, total volume, mixing, drop size and endpoint judgement.

Use a water bath rather than a flame because ethanol and many halogenoalkanes are volatile and flammable; minimise quantities, avoid inhalation and dispose of silver/halogenated waste appropriately.

The tertiary isomer gives the silver-halide precipitate first, the secondary next and the primary last when concentration, halogen, temperature and mixing are controlled.

This is the empirical AS trend for these hydrolysis conditions. Later SN1/SN2 study explains why mechanism and solvent can alter structural effects; here, use the observed order without claiming every nucleophilic substitution follows it.

Hydrolysis requires C–X bond breaking. Less energy is needed to break C–I than C–Br or C–Cl, so otherwise comparable iodoalkanes react fastest: RCl < RBr < RI.

In the silver nitrate test, AgI appears first, AgBr next and AgCl last for matched primary compounds. Precipitate colour separately confirms the halide.

2-methylpropan-2-ol reacts with concentrated hydrochloric acid to form 2-chloro-2-methylpropane and water: (CH$_3$)$_3$COH+HCl$\rightarrow$(CH$_3$)$_3$CCl+H$_2$O.

Identify layers by density or a drop test rather than assuming top/bottom, minimise transfer losses, and assess purity from a narrow boiling range. Concentrated HCl is corrosive and the product is volatile/flammable: use ventilation and no flame.

Choose the longest chain containing the carbon bonded to –OH. Number from the end giving –OH the lowest locant, replace the alkane -e with -ol, and then add and alphabetise substituent prefixes.

Structural formulae show connectivity, displayed formulae show every bond, and skeletal formulae show carbon vertices while the O and H of –OH must be written explicitly.

Locate the carbon directly bonded to oxygen in –OH, then count the other carbons directly bonded to that carbon. Hydrogens and the O atom are not counted as carbon groups.

Classification predicts oxidation: primary alcohols form aldehydes/acids, secondary form ketones, and tertiary alcohols resist oxidation under ordinary acidified dichromate conditions.

For ethanol combustion: C$_2$H$_5$OH+3O$_2$$\rightarrow$2CO$_2$+3H$_2$O. For dehydration: CH$_3$CH$_2$OH$\rightarrow$CH$_2$=CH$_2$+H$_2$O.

Know reagents, conditions, products and observations; mechanisms for these alcohol reactions are not required here.

CH$_3$CH$_2$CH$_2$OH+[O]$\rightarrow$CH$_3$CH$_2$CHO+H$_2$O; then CH$_3$CH$_2$CHO+[O]$\rightarrow$CH$_3$CH$_2$COOH. Propan-2-ol+[O]$\rightarrow$propanone+H$_2$O.

Distillation removes the volatile aldehyde before further oxidation. Reflux returns vapour for sustained contact with excess oxidant, favouring the acid.

A typical preparation uses reflux or controlled distillation for reaction, separating-funnel washes/extraction, drying of the retained organic layer, then final distillation to collect the boiling fraction.

Never seal a heated/distillation system, add anti-bumping granules before heating, vent a separating funnel away from people, and use non-flame heating for flammable liquids.

Use anti-bumping granules and controlled addition/heating. For propanal, a sealed-left/open-receiver distillation setup has a correctly placed thermometer and downward condenser with water in at the bottom; collect the appropriate boiling fraction.

Dichromate changes orange to green. Confirm propanal with Benedict's/Fehling's brick-red precipitate and propanoic acid by CO$_2$ effervescence with carbonate/hydrogencarbonate; boiling range supports purity.

Acidified dichromate(VI) is toxic/oxidising and sulfuric acid corrosive; use small scale, eye protection and suitable waste. Organic vapours are flammable, so avoid naked flames.

For a singly charged ion, $m/z$ equals its relative ionic mass. The molecular-ion peak M$^{+\boldsymbol{\cdot}}$ gives the molecular relative mass; fragment peaks arise when it breaks, and the base peak is the most intense ion.

For C$_3$H$_8$O with M$^+$ at 60, a strong $m/z=31$ ion such as CH$_2$OH$^+$ supports propan-1-ol, while a strong $m/z=45$ fragment supports cleavage patterns of propan-2-ol. The same molecular ion alone cannot distinguish isomers.

Use the supplied wavenumber table, identify strong/broad diagnostic absorptions, combine features into functional groups, and use meaningful absences to eliminate candidates. Predict a spectrum by listing every characteristic bond in the structure.

Broad alcohol O–H without C=O supports an alcohol; very broad acid O–H plus strong C=O supports a carboxylic acid; strong C=O without O–H could be an aldehyde or ketone and needs C–H/mass evidence.

Plan a branching sequence so one test does not contaminate the next, use fresh portions and blanks, record reagent/condition/observation, and require at least two compatible pieces of evidence before naming an unknown.

Risk-assess corrosive acids/alkalis, oxidising dichromate, volatile organics and silver waste; work microscale with ventilation and segregated disposal.