Difficulty: Intermediate | Prerequisites: Ch. 1–4 (structure, bonding, chirality)
Spectroscopy (Ch. 14) gives you the tools to figure out what a molecule looks like without seeing it directly: IR tells you which functional groups are present, and NMR tells you the carbon-hydrogen framework. Alcohols and alkyl halides (Ch. 5) are the first functional-group families you study in depth, and they set up the substitution and elimination reactions in the chapters that follow.
Infrared (IR) spectroscopy
A technique that measures the absorption of infrared light by molecular bonds. Different bonds absorb at characteristic frequencies (measured in wavenumbers, cm-1), so an IR spectrum tells you which functional groups are in the molecule.
Wavenumber (cm-1)
The unit used in IR spectroscopy, equal to 1/wavelength in centimetres. Higher wavenumber = higher energy = stronger or lighter bond.
Functional group region (4000-1500 cm-1)
The part of an IR spectrum where distinctive absorptions appear: O-H, N-H, C-H stretches (3000-3600 cm-1), carbonyl C=O stretch (around 1700 cm-1), C=C stretch (around 1650 cm-1).
Fingerprint region (1500-400 cm-1)
The complex region of an IR spectrum that is unique to each molecule, like a fingerprint. Useful for confirming identity by comparison, but hard to interpret peak by peak.
Nuclear magnetic resonance (NMR) spectroscopy
A technique that exploits the magnetic properties of certain nuclei (1H, 13C) to reveal the structural environment of atoms in a molecule. The most powerful structure-determination tool in organic chemistry.
Chemical shift (delta, ppm)
The position of an NMR signal relative to a reference compound (TMS, tetramethylsilane = 0 ppm). Electronegative neighbours shift signals downfield (higher ppm). In simple terms, chemical shift tells you what electronic environment an atom sits in.
Shielding and deshielding
Shielded nuclei are surrounded by electron density and resonate upfield (low ppm). Deshielded nuclei have electron density pulled away (by electronegative atoms, pi systems) and resonate downfield (high ppm).
Splitting (spin-spin coupling)
The pattern of peaks in a 1H NMR signal caused by neighbouring non-equivalent hydrogens. Follows the n+1 rule: a hydrogen with n equivalent neighbours gives n+1 peaks (singlet, doublet, triplet, quartet, etc.).
Integration
The area under each 1H NMR signal, proportional to the number of hydrogens producing that signal.
Alcohol
An organic compound containing a hydroxyl (-OH) group bonded to an sp3 carbon. Classified as primary (1 degree), secondary (2 degree), or tertiary (3 degree) based on the substitution of the carbon bearing the OH.
Alkyl halide
An organic compound in which a halogen (F, Cl, Br, or I) is bonded to an sp3 carbon. Also classified as primary, secondary, or tertiary.
Primary, secondary, tertiary (classification)
Refers to the number of carbon atoms directly attached to the carbon bearing the functional group. Primary = one carbon neighbour, secondary = two, tertiary = three. This classification is critical for predicting reactivity in substitution and elimination reactions.
A molecule absorbs IR radiation when the frequency matches the vibrational frequency of one of its bonds. Only bonds with a changing dipole moment absorb (symmetric stretches of non-polar bonds, like the C=C in ethene, give weak or absent signals).
Key absorptions to memorise:
O-H stretch: broad, strong, 3200-3600 cm-1 (alcohols); very broad 2500-3300 cm-1 (carboxylic acids)
N-H stretch: medium, 3300-3500 cm-1 (one peak for secondary amines, two for primary)
C-H stretch: just below 3000 cm-1 for sp3 C-H; just above 3000 cm-1 for sp2 C-H; around 3300 cm-1 for sp C-H (terminal alkyne)
C=O stretch: strong, sharp, around 1700 cm-1 (the most recognisable absorption in IR)
C=C stretch: medium, around 1650 cm-1
C-O stretch: strong, 1000-1260 cm-1
Strategy for reading an IR spectrum: scan left to right. Check for O-H or N-H (3200-3600 cm-1), check C-H region (sp3 vs. sp2), check for carbonyl (1700 cm-1), then note anything else distinctive.
Protons in different electronic environments give signals at different chemical shifts. Equivalent protons give one signal.
Predicting number of signals: identify all sets of equivalent hydrogens by symmetry. Two hydrogens are equivalent if they can be interchanged by a symmetry operation (rotation, mirror plane).
Typical chemical shift ranges:
Alkyl C-H (R-CH3, R2-CH2, R3-CH): 0.5-2.0 ppm
Allylic (next to C=C): around 2.0 ppm
C-H next to electronegative atom (O, N, X): 2.5-4.5 ppm
Vinyl C-H (on C=C): 4.5-6.5 ppm
Aromatic C-H: 6.5-8.5 ppm
Aldehyde C-H: around 9.5 ppm
Carboxylic acid O-H: 10-12 ppm
Alcohol O-H: variable, often 1-5 ppm (broad)
Splitting patterns follow the n+1 rule. A proton with n equivalent neighbours on adjacent carbons appears as (n+1) peaks. The coupling constant J (in Hz) is the same for both coupled signals.
Integration gives the relative number of protons for each signal. If total integration corresponds to, say, 10 units and you know there are 10 H atoms, each unit is one H.
Each unique carbon environment gives one peak. No splitting in routine broadband-decoupled 13C spectra (all peaks appear as singlets).
Chemical shift ranges mirror 1H but span a wider range (0-220 ppm):
Alkyl carbons: 0-50 ppm
Carbons bonded to O or N: 50-100 ppm
Alkene and aromatic carbons: 100-160 ppm
Carbonyl carbons: 160-220 ppm
13C NMR does not give integration information in routine spectra (peak heights are not proportional to number of carbons). DEPT experiments distinguish CH3, CH2, CH, and quaternary C.
IUPAC naming of alcohols: find the longest chain containing the OH group, change the -e ending to -ol, number from the end nearest the OH. The OH takes priority over halogens and alkyl branches in numbering.
Common names: methanol (methyl alcohol), ethanol (ethyl alcohol), 2-propanol (isopropyl alcohol), etc.
Alkyl halides: named as haloalkanes (IUPAC) or as alkyl + halide (common). Fluoro-, chloro-, bromo-, iodo- are treated as substituents in IUPAC naming.
Classification as primary, secondary, or tertiary is based on the carbon bearing the functional group, not the molecule as a whole.
Alcohols have much higher boiling points than alkyl halides or alkanes of similar molecular weight because of hydrogen bonding between O-H groups.
Low-molecular-weight alcohols (methanol, ethanol, propanol) are miscible with water. Solubility decreases as the hydrocarbon portion grows.
Alkyl halides are denser than most organic compounds (especially bromo and iodo compounds). They are not soluble in water.
Reaction with hydrogen halides (HX): the OH is replaced by X. Reactivity order: HI > HBr > HCl > HF. Tertiary alcohols react fastest (via carbocation intermediate, SN1). Primary alcohols react slowest and may need a Lewis acid catalyst (ZnCl2) or an SN2 pathway.
Reaction with thionyl chloride (SOCl2): converts R-OH to R-Cl with inversion of configuration at a chiral centre (SN2-like mechanism). Produces SO2 and HCl as gaseous by-products, which drive the reaction to completion.
Reaction with phosphorus tribromide (PBr3): converts R-OH to R-Br. Works well for primary and secondary alcohols. SN2 mechanism with inversion.
Carbocation stability: tertiary > secondary > primary > methyl. More substituted carbocations are stabilised by hyperconjugation (donation of electron density from adjacent C-H sigma bonds into the empty p orbital).
Carbocation rearrangements: when a less stable carbocation can become a more stable one by a 1,2-shift (a hydride shift or a methyl shift), it will. This can lead to unexpected products.
Look for rearrangement whenever a reaction goes through a carbocation intermediate and the initial carbocation is secondary or primary with a more substituted carbon adjacent.
The C-O bond in alcohols and the C-X bond in alkyl halides are both polar. The carbon is electrophilic (electron-poor) and can be attacked by nucleophiles.
Bond strength decreases down the halogen group: C-F > C-Cl > C-Br > C-I. This means C-I bonds are easiest to break, making iodide the best leaving group among the halogens.
The concept of leaving group ability: a good leaving group is a weak base. The halide ions rank as leaving groups: I- > Br- > Cl- >> F-. Hydroxide (OH-) is a poor leaving group, which is why alcohols must be activated (protonated or converted to a sulfonate ester) before substitution.
Key IR absorptions to memorise
O-H (alcohol): broad, 3200-3600 cm-1
O-H (carboxylic acid): very broad, 2500-3300 cm-1
N-H: 3300-3500 cm-1
sp3 C-H: just below 3000 cm-1
sp2 C-H: just above 3000 cm-1
C triple bond C-H (terminal alkyne): around 3300 cm-1, sharp
C=O: strong, sharp, around 1700 cm-1
C=C: medium, around 1650 cm-1
1H NMR splitting: the n+1 rule
Number of peaks = n + 1, where n is the number of equivalent neighbouring hydrogens on adjacent carbons.
Singlet (s) = 0 neighbours, doublet (d) = 1, triplet (t) = 2, quartet (q) = 3, quintet = 4, sextet = 5, septet = 6.
Carbocation stability order
Tertiary (3 degree) > secondary (2 degree) > primary (1 degree) > methyl
Leaving group ability (halides)
I- > Br- > Cl- >> F-
Correlates inversely with basicity and directly with the weakness of the C-X bond.
NMR spectroscopy is the workhorse of pharmaceutical development and quality control. Every new drug candidate is characterised by 1H and 13C NMR to confirm its structure. MRI (magnetic resonance imaging) in hospitals is the same underlying physics, applied to hydrogen atoms in water and fat in the body.
Alkyl halides are widely used as solvents (dichloromethane, chloroform) and as starting materials in organic synthesis. Chlorofluorocarbons (CFCs), once common refrigerants, are alkyl halides whose environmental impact led to the Montreal Protocol banning their production.
Students often think that the number of peaks in a 1H NMR spectrum equals the number of hydrogen atoms. It does not. It equals the number of sets of chemically non-equivalent hydrogens. A molecule with 10 H atoms might show only 3 signals if many are equivalent.
Students frequently confuse the n+1 rule with the number of hydrogens on the carbon itself. The splitting is caused by hydrogens on neighbouring carbons, not the same carbon.
A common error is assuming OH protons always show splitting. In practice, O-H signals are often broad singlets because rapid proton exchange with traces of water averages out the coupling.
Students sometimes think that a stronger C-X bond means a better leaving group. The opposite is true: weaker bonds break more easily, making I- a better leaving group than F- even though C-F is the strongest carbon-halogen bond.
⚠️ IR interpretation is tested as "identify the functional groups present in this spectrum." Memorise the five or six key absorptions (O-H, N-H, C-H, C=O, C=C, C triple bond C) and their approximate positions.
⚠️ 1H NMR problem-solving (given a molecular formula and a spectrum, determine the structure) is one of the most common and highest-value exam questions. Practise by working backwards from spectra.
⚠️ Carbocation rearrangements catch students off guard. Any time a reaction forms a carbocation, ask: "Is there a more stable carbocation one shift away?" If yes, expect the product to reflect the rearranged carbocation.
⚠️ Know the three reagents for converting alcohols to alkyl halides (HX, SOCl2, PBr3) and what stereochemical outcome each gives.
True or false: A broad absorption around 3300 cm-1 in an IR spectrum could be either an O-H or an N-H stretch. (True: you need additional information, like the molecular formula, to distinguish them.)
Fill in the blank: A proton that appears as a triplet in 1H NMR has ______ equivalent neighbours on adjacent carbons. (Two)
True or false: 13C NMR peaks in a routine broadband-decoupled spectrum appear as multiplets. (False: they appear as singlets.)
Fill in the blank: The reagent SOCl2 converts an alcohol to a ______ with ______ of configuration at a chiral centre. (Chloride, inversion)
True or false: A primary carbocation is more stable than a secondary carbocation. (False: secondary is more stable than primary.)
Q: A compound with molecular formula C4H8O shows a broad absorption at 3350 cm-1 and no absorption near 1700 cm-1 in its IR spectrum. Its 1H NMR shows a triplet at 3.7 ppm (2H), a broad singlet at 2.3 ppm (1H), a sextet at 1.6 ppm (2H), and a triplet at 0.9 ppm (3H). What is the structure?
A: The broad absorption at 3350 cm-1 and no carbonyl peak suggest an alcohol (not an aldehyde or ketone). The molecular formula gives one degree of unsaturation (0, so no rings or double bonds). The NMR pattern (triplet-sextet-triplet for CH2-CH2-CH3 plus an OH singlet) is consistent with 1-butanol (CH3CH2CH2CH2OH).
Q: Predict the major product when 2-methyl-2-butanol is treated with HBr.
A: The tertiary alcohol is protonated to form a good leaving group (water), which departs to form a tertiary carbocation. Bromide attacks to give 2-bromo-2-methylbutane. No rearrangement occurs because the carbocation is already tertiary (the most stable option).
Q: What product would you expect from treating 3,3-dimethyl-2-butanol with HBr? Consider rearrangement.
A: Protonation and loss of water initially gives a secondary carbocation at C2. A methyl shift from C3 to C2 converts this to a more stable tertiary carbocation at C3. Bromide then attacks C3, giving 2-bromo-2,3-dimethylbutane as the major product.
Q: In the 1H NMR spectrum of diethyl ether (CH3CH2-O-CH2CH3), how many signals do you expect, and what splitting pattern does each show?
A: Two signals. The CH3 group (6H) appears as a triplet (split by the adjacent CH2). The CH2 group (4H) appears as a quartet (split by the adjacent CH3). The molecule has a plane of symmetry, so both ethyl groups are equivalent.
Spectroscopy appears in every subsequent chapter as the method for confirming the identity of reaction products. From this point forward, exam questions may give you a spectrum and ask you to identify a product, or ask you to predict what spectroscopic features a product would show.
The concepts of leaving groups and carbocation stability from Chapter 5 are the direct foundation for nucleophilic substitution (Ch. 6) and elimination (Ch. 7). If you understand why tertiary carbocations form easily and why iodide is a good leaving group, the mechanism discussions in those chapters will make sense.
Hydrogen bonding in alcohols connects back to intermolecular forces from general chemistry and forward to the physical properties of every oxygen- or nitrogen-containing functional group you will encounter.
IR spectroscopy, infrared spectrum, wavenumber, cm-1, functional group region, fingerprint region, O-H stretch, C=O stretch, carbonyl absorption, NMR spectroscopy, proton NMR, 1H NMR, 13C NMR, carbon NMR, chemical shift, ppm, delta, shielding, deshielding, downfield, upfield, TMS, tetramethylsilane, splitting pattern, n+1 rule, coupling constant, J value, integration, singlet, doublet, triplet, quartet, DEPT, alcohol, hydroxyl group, alkyl halide, haloalkane, primary secondary tertiary, 1 degree 2 degree 3 degree, hydrogen bonding, boiling point, leaving group, nucleophile, carbocation, carbocation rearrangement, hydride shift, methyl shift, hyperconjugation, SOCl2 thionyl chloride, PBr3 phosphorus tribromide, HBr HCl HI, SN1 SN2, inversion of configuration, Carey organic chemistry chapter 5, chapter 14, CHEM 2301 UMN