Chemistry · Organic chemistry · Grade 10

Organic chemistry: structure

Why does burnt sugar leave charcoal, and why does carbon form four bonds?

The situation

If you forget sugar in a small pan over a flame, it first melts, then darkens and turns to charcoal, and drops of water settle on the sides of the pan. However long you heat table salt, it does not darken. Sugar, paraffin wax, fat, protein — these are all organic substances: they always contain carbon, and almost always hydrogen. 200 years ago scientists thought that only a living organism could create such substances. In what order do carbon atoms join together, and how can we prove that a substance contains carbon and hydrogen?

Topic

Introduction to organic chemistry: structural theory. Wöhler's urea and Butlerov's theory: burnt sugar leaves charcoal — detecting carbon and hydrogen in an organic substance

Key ideas

Butlerov's theory of structure
Carbon tetrahedron (109.5°)
Detecting C and H

Formulas

NH_{4} OCN \overset{\overset{}{t}}{\to} CO (NH_{2})_{2}

Wöhler (1828): organic urea from inorganic ammonium cyanate — the composition is the same (CH₄N₂O), the structure is different.

C_{12} H_{22} O_{11} + 24 CuO \overset{\overset{}{t}}{\to} 12 CO_{2} ↑ + 11 H_{2} O + 24 Cu

Composition analysis: copper(II) oxide oxidises sugar to CO₂ and water, and is itself reduced to red copper.

Ca (OH)_{2} + CO_{2} \overset{}{\to} CaCO_{3} ↓ + H_{2} O

Limewater turns cloudy — CO₂ is present, so the substance contains carbon.

CuSO_{4} (white) + 5 H_{2} O \overset{}{\to} CuSO_{4} \cdot 5 H_{2} O (blue)

Anhydrous copper sulfate turns blue — water has formed, so the substance contains hydrogen.

∠ H - C - H = {109.5}^{\circ}, l (C - C) = 154 pm

sp³ carbon: the four bonds point to the corners of a tetrahedron.

Theory

Organic chemistry and its history (1.1)

Organic chemistry is the chemistry of carbon compounds (CO, CO₂, carbonates, carbides and cyanides are usually studied in inorganic chemistry). At the beginning of the 19th century Berzelius gave this name to substances obtained from living organisms; the doctrine of vitalism held that only a “vital force” creates them. In 1828 F. Wöhler heated inorganic ammonium cyanate and obtained organic urea; then Kolbe synthesised ethanoic (acetic) acid (1845), and Berthelot fats and ethyne (acetylene) — and vitalism lost its force. Properties of organic substances: mostly covalent bonds; most melt and boil at low temperatures, burn, and char when heated; their reactions are slower; there are many substances with the same composition but different structures (isomers). Most of the substances known today are organic: the CAS registry holds more than 200 million substances. In Uzbekistan in 1956 Academician S. Yu. Yunusov founded the Institute of the Chemistry of Plant Substances, where plant alkaloids are studied; the A. S. Sadikov Institute of Bioorganic Chemistry is also in Tashkent.

Butlerov's theory of structure (1.2)

In 1861 A. M. Butlerov gave a lecture “On the chemical structure of substances” at the congress in Speyer (Kekulé and Couper had expressed the tetravalency of carbon in 1858). The main principles: 1) the atoms in a molecule are joined in a definite order according to their valency — this order is the chemical structure; carbon is tetravalent and forms chains and rings with itself. 2) The properties of a substance depend not only on its composition but also on its structure — which is why C₂H₆O is two different substances: ethanol (a liquid, 78.3 °C) and dimethyl ether (a gas, −24.8 °C). 3) From the properties one can determine the structure and, conversely, predict the properties from the structure. 4) Atoms influence one another; those directly bonded do so more strongly. Relying on his theory, Butlerov synthesised isobutane (1866) and tert-butyl alcohol. In 1874 van 't Hoff and Le Bel showed that the four bonds of carbon point to the corners of a tetrahedron: the angle is 109.5°, C–C 154 pm, C–H 109 pm (the models in the scene have these dimensions).

Valency and oxidation state (1.3)

In organic compounds carbon is always tetravalent (four shared electron pairs), but its oxidation state varies from −4 to +4. The counting rule: each C–H bond of the carbon gives −1 (hydrogen is less electronegative), each C–O, C–N or C–Cl bond gives +1 (a multiple bond counts as many times as its order), and a C–C bond gives 0. For example: CH₄ (−4), CH₃OH (−2), HCHO (0), HCOOH (+2), CO₂ (+4) — this series is the stepwise oxidation of carbon; combustion is its last step. The carbons in one molecule can differ: in ethanol CH₃ is −3 and CH₂OH is −1. The average value for the molecule also follows from the formula: for C₂H₆O, 2x + 6 − 2 = 0, x = −2 (the average of −3 and −1).

Practical: comparing organic substances and determining their composition (1.9–1.10)

Comparison: sugar and paraffin wax melt when heated, then char or burn; table salt does not change up to 801 °C. Qualitative analysis: a mixture of ≈ 0.1 g of the substance and 1–2 g of black copper(II) oxide powder is put into a dry test tube, which is clamped with its mouth tilted slightly downwards (so that the water formed does not run back onto the hot glass). Anhydrous CuSO₄ is sprinkled on a piece of cotton wool near the mouth; the delivery tube is lowered into limewater. When heated, CuO oxidises the substance: carbon becomes CO₂ — the limewater turns cloudy; hydrogen becomes water — the CuSO₄ turns blue and drops appear on the wall; the black CuO is reduced to red copper. If CO₂ is passed for a long time, the cloudiness disappears again: $CaCO_{3} + CO_{2} + H_{2} O \overset{}{\to} Ca (HCO_{3})_{2}$ . Quantitative analysis (Liebig's method): the water is completely absorbed in a CaCl₂ tube and the CO₂ in a KOH tube; from the increase in mass of the tubes, $m (C) = Δ m (C O_{2}) \cdot \frac{12}{44}$ , $m (H) = Δ m (H_{2} O) \cdot \frac{2}{18}$ , and oxygen is found by difference; the molar ratio gives the empirical formula. Safety: wear safety glasses; before stopping the heating, take the tube out of the limewater, otherwise the cold liquid will be sucked into the hot test tube and it will crack; never point the mouth of the test tube at anyone.

Electronic effects and reaction mechanisms (University)

Butlerov's 4th principle in modern language is electronic effects. The inductive effect (I) is the shift of electron density along a chain of σ-bonds towards the more electronegative atom; −I: F > Cl > Br > I, OH, NO₂; +I: alkyl groups. It weakens by a factor of about 2–3 with each bond: the pKa values of 2-, 3- and 4-chlorobutanoic acids are 2.86, 4.05 and 4.52. The more stable the conjugate anion of an acid, the stronger the acid: CH₃COOH 4.76 → ClCH₂COOH 2.87 → Cl₃CCOOH 0.66. The mesomeric effect (M) is the shift of π-electrons and lone pairs along a conjugated system. In phenol the oxygen lone pair is conjugated with the benzene ring (+M), and the charge of the phenoxide anion is spread over the ring — phenol (pKa 9.99) is a much stronger acid than ethanol (≈ 16); in aniline the nitrogen lone pair is “given” to the ring, so aniline (pKa of BH⁺ 4.6) is a weaker base than ammonia (9.25). Classification of mechanisms: if a bond breaks homolytically (one electron to each atom), free radicals form and radical reactions occur (chlorination of methane in light); if it breaks heterolytically, ions form: a species that accepts an electron pair is an electrophile (H⁺, Br⁺, NO₂⁺), and one that donates a pair is a nucleophile (OH⁻, NH₃, H₂O); electrophilic addition (alkenes), nucleophilic substitution (haloalkanes).

About the model

The molecules are built with true bond lengths and angles (C–C 154 pm, C=C 134 pm, C–H 109 pm, sp³ 109.5°, sp² 120°) and magnified ≈ 5·10⁸ times; the atom spheres are shrunk by convention (“ball-and-stick”), and in the “space-filling” view van der Waals radii are shown. The conformations are the most favourable “zigzag”. In the analysis model the heating and reaction times are shortened; the fraction of CO₂ absorbed by limewater and the capacity of CuSO₄ are approximate (qualitatively correct), and in the quantitative analysis absorption is taken as complete. In the animation of urea formation the atoms “fly across” — the real mechanism is stepwise: $NH_{4}^{+} + OCN^{-} \overset{}{⇌} NH_{3} + HNCO$ , then ammonia adds to HNCO. The pKa values are in water at 25 °C (CRC Handbook).

Did you know?

In February 1828 Wöhler wrote to his teacher Berzelius that he had made urea “without needing a kidney or any animal”.
More than 200 million substances are recorded in the CAS registry, and most of them are carbon compounds; thousands of new ones are added every day.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

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Alcohols and ethers — Grade 10
Phenol, aldehydes and ketones — Grade 10
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