Organic Chemistry – Some Basic Principles and Techniques: common doubts, answered
The questions students ask most often about Organic Chemistry – Some Basic Principles and Techniques, each with a short answer. For the full chapter, read the Organic Chemistry – Some Basic Principles and Techniques notes.
Carbon's tetravalence and the shapes of organic molecules
Read this section in the notes →Why is an sp hybridised carbon more electronegative than an sp³ carbon?
The more s character a hybrid orbital has, the closer its electrons stay to the nucleus, so the carbon attracts bonding electrons more strongly. An sp orbital is 50% s, sp² is 33% and sp³ is 25%, so electronegativity runs sp > sp² > sp³. This is why the hydrogen of ethyne is weakly acidic while that of ethane is not.
Structural representations
Read this section in the notes →How do you read a bond-line structure?
In a bond-line structure carbon and hydrogen atoms are not written out. Each line is a bond, and every line end and every corner where lines meet is a carbon atom, which is assumed to carry enough hydrogens to make four bonds. Other atoms, such as O, N or Cl, are always shown. A plain zigzag of four lines therefore represents pentane.
Classification of organic compounds
Read this section in the notes →What is a homologous series?
A homologous series is a family of compounds that share a functional group, with each member differing from the next by one –CH₂– unit. Members fit one general formula, such as CₙH₂ₙ₊₁OH for alcohols, react in similar ways and show a gradual change in physical properties such as boiling point. Methanol, ethanol and propanol belong to one such series.
IUPAC nomenclature
Read this section in the notes →How do you choose and number the parent chain in IUPAC naming?
Pick the longest continuous chain of carbon atoms, even if it bends on paper, and make sure it contains the principal functional group and any multiple bond. Number it from the end that gives the lowest locant first to the principal group, then to multiple bonds, then to substituents. Counting all the carbons in the molecule, branches included, as the chain is a classic error.
Which functional group gets priority in IUPAC naming?
The group highest on the priority list becomes the suffix and the rest become prefixes. The order runs –COOH > –SO₃H > –COOR > –COCl > –CONH₂ > –CN > –CHO > >C=O > –OH > –NH₂, with double and triple bonds below these. So HOCH₂CH₂COOH is named as an acid, 3-hydroxypropanoic acid. Halo, nitro and alkoxy groups are only ever prefixes.
How are substituents arranged alphabetically in an IUPAC name?
Prefixes for substituents are written in alphabetical order of their names, and multiplying prefixes such as di, tri and tetra are ignored when sorting. So ethyl is written before dimethyl, because e is compared with m and the di is skipped. The alphabet decides only the order of writing; locant numbers are still chosen to give the lowest possible set.
Isomerism
Read this section in the notes →What is metamerism?
Metamerism is isomerism that arises from different alkyl groups sitting on either side of a functional group, such as the oxygen of an ether. Methoxypropane, CH₃–O–C₃H₇, and ethoxyethane, C₂H₅–O–C₂H₅, both have the formula C₄H₁₀O but divide their carbons differently around the oxygen. It is one kind of structural isomerism, along with chain, position and functional group isomerism.
Bond cleavage and reactive intermediates
Read this section in the notes →What is the difference between homolytic and heterolytic cleavage?
In homolytic cleavage a covalent bond splits evenly, each atom keeping one of the shared electrons, so two free radicals form; heat or light in non-polar conditions favour it. In heterolytic cleavage one atom keeps both electrons, giving a cation and an anion, such as a carbocation or a carbanion, and polar solvents favour it. Half-headed arrows track single electrons, full arrows pairs.
Why is a tertiary carbocation more stable than a primary one?
Alkyl groups feed electron density towards the positive carbon through their +I effect and through hyperconjugation from their C–H bonds. A tertiary carbocation has three alkyl groups and the most α-hydrogens, so its positive charge is spread out the most. That gives the order 3° > 2° > 1° > CH₃⁺, and explains why tertiary substrates react fastest by routes involving carbocations.
What is the shape of a carbocation and a carbanion?
In a carbocation the charged carbon is sp² hybridised, with three bonds in one plane and an empty p orbital, so it is trigonal planar. In a carbanion the carbon is sp³ hybridised, with three bonds and a lone pair in the fourth hybrid orbital, so it is pyramidal, like ammonia. Assuming both are planar is a common mistake.
What is the difference between an electrophile and a nucleophile?
A nucleophile is electron-rich and donates an electron pair to an electron-poor centre; OH⁻, CN⁻ and the lone pair of NH₃ are examples. An electrophile is electron-poor and accepts an electron pair; carbocations are examples, and so are neutral molecules like BF₃ and AlCl₃, whose central atom lacks an octet. In a polar reaction the nucleophile attacks the electrophile.
Electron displacement effects
Read this section in the notes →What is the inductive effect and why does it fade along a carbon chain?
The inductive effect is a permanent drift of σ-bond electrons towards a more electronegative atom or group. Each bond passes on only part of the polarisation it receives, so the effect is felt on the nearest one or two carbons and becomes negligible beyond about three bonds. Halogens withdraw electrons in the order F > Cl > Br > I, while alkyl groups donate them.
What is the difference between the inductive effect and the electromeric effect?
The inductive effect is permanent and operates through σ bonds, while the electromeric effect is temporary and acts on a multiple bond. When an attacking reagent comes near, the π electrons of a C=C or C=O shift completely onto one atom; when the reagent is removed, the molecule returns to normal. So the electromeric effect exists only while a reagent is present.
What is hyperconjugation?
Hyperconjugation is the spreading of electrons from a C–H σ bond on a carbon next to a double bond or an empty p orbital into that neighbouring orbital. The delocalisation stabilises carbocations, free radicals and alkenes, and the more α-hydrogens there are, the larger the effect. Because the drawn structures show no bond between that hydrogen and carbon, it is also called no-bond resonance.
Resonance in organic molecules
Read this section in the notes →What is the difference between +R and −R effects?
In a +R effect a group pushes electron density into a conjugated system, usually by sharing a lone pair, as –OH, –NH₂ and halogens do on a benzene ring. In a −R effect a group pulls electron density out through its own π bond, as –NO₂, –CHO and –CN do. Resonance works through π electrons and is not limited to nearby atoms like the inductive effect.
Why does resonance make a molecule more stable?
Resonance spreads electrons over several atoms, and delocalised electrons sit at lower energy than electrons confined to one bond. The actual molecule is a single hybrid that is more stable than any of the structures drawn for it, and the difference is called the resonance energy. Benzene shows this: all its C–C bonds are 139 pm long, and it resists the addition reactions typical of alkenes.
Types of organic reactions
Read this section in the notes →What are the main types of organic reactions?
There are four broad types. In substitution one atom or group replaces another; in addition atoms join across a multiple bond; in elimination atoms are removed from neighbouring carbons to create a multiple bond; and in rearrangement atoms within a molecule shift to give an isomer. Identifying the type early helps you predict the product and choose suitable reagents.
Methods of purification
Read this section in the notes →When do you use steam distillation, fractional distillation or distillation under reduced pressure?
Simple distillation works when boiling points differ widely. Fractional distillation, with a fractionating column, is needed when they are close, as in refining crude oil. Liquids that decompose at their normal boiling point are distilled under reduced pressure, such as glycerol from spent-lye. Steam distillation suits compounds that are steam-volatile and immiscible with water, such as aniline.
How does chromatography separate a mixture and what is Rf?
Chromatography works because the components of a mixture divide themselves differently between a fixed stationary phase and a moving mobile phase. Components held less firmly by the stationary phase travel farther. In thin layer or paper chromatography each spot is described by its retardation factor, Rf = distance moved by the substance / distance moved by the solvent front, so Rf is always below 1.
Qualitative analysis: detecting elements
Read this section in the notes →Why is the compound fused with sodium in Lassaigne's test?
Nitrogen, sulphur and halogens in organic compounds are held by covalent bonds and do not respond to ordinary ionic tests. Fusing with sodium converts them into sodium cyanide, sodium sulphide and sodium halides, which dissolve in water and can be tested. Before adding AgNO₃ for halogens, the extract is boiled with nitric acid to remove cyanide and sulphide, which would also precipitate with silver ions.
Quantitative analysis: estimating elements
Read this section in the notes →What is the difference between the Dumas and Kjeldahl methods, and why does Kjeldahl fail for nitrobenzene?
The Dumas method heats the compound with copper oxide and measures its nitrogen as N₂ gas, so it works for all nitrogen compounds. The Kjeldahl method digests the compound in concentrated sulphuric acid to ammonium sulphate and estimates the ammonia released. Nitrogen in nitro and azo groups, or within a ring as in pyridine, does not convert to ammonium sulphate, so Kjeldahl gives wrong results there.
How does the Carius method estimate halogens and sulphur?
The compound is heated with fuming nitric acid in a sealed Carius tube. For halogens, silver nitrate is included, and the halogen is collected and weighed as silver halide. For sulphur, the element is oxidised to sulphuric acid and then precipitated as barium sulphate by adding barium chloride. The mass of AgX or BaSO₄ obtained gives the percentage of the element.
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