☰ Contents · Chemistry (Organic chemistry)

History of organic chemistry and the structure theory

Lessons 1–2 · 2 lessons · A. Mutalibov, E. Murodov, S. Masharipov, H. Islomova. Organic Chemistry, Grade 10, 1st edition. G‘afur G‘ulom Publishing House, Tashkent, 2017
2

The theory of chemical structure of organic substances

Textbook: pp. 7–10
GoalState the main points of Butlerov’s theory of chemical structure and calculate carbon oxidation states in organic compounds.
New words
chemical structure · kimyoviy tuzilishvalence · valentlikoxidation state · oksidlanish darajasimutual influence of atoms · atomlarning o‘zaro ta’siri
Explanation

According to Butlerov’s theory, the atoms in a molecule are joined in a definite order according to their valences; this order is the chemical structure. In organic compounds carbon is tetravalent, oxygen divalent, nitrogen trivalent and hydrogen monovalent. Properties depend not only on composition but also on the order of bonding: C₂H₅OH (ethanol) and CH₃–O–CH₃ (dimethyl ether) both have the formula C₂H₆O yet differ completely. Atoms and groups in a molecule influence each other: the –OH group gives a neutral medium in C₂H₅OH, a strong base in NaOH and acidity in CH₃COOH. Carbon’s valence is always IV, but its oxidation state ranges from −4 to +4: in a C–H bond the shared pair shifts toward carbon (electronegativity 2.5 vs 2.1 for hydrogen), while in a C–C bond it does not shift. So carbon is −4 in methane, −2 in methanol, 0 in methanal, +2 in formic acid and +4 in CO₂.

Worked examples
Propane CH₃–CH₂–CH₃: each end carbon has 3 H (−3), the middle one has 2 H (−2). Sum: −3 − 2 − 3 = −8.
Acetic acid CH₃COOH: the C in CH₃ is −3; the C in COOH is bonded to O twice (=O counts +2) and once through –OH (+1) and to C (0): +3. Sum for both carbons: −3 + 3 = 0.
Class activity

With plasticine and toothpicks (or coloured paper and string) build two molecule models of C₂H₆O: one ethanol, one dimethyl ether. Check that every carbon has four bonds and oxygen has two. Handle toothpicks carefully, with an adult’s help.

Practice
1
What valences do C, H and O show in organic compounds?
2
How many moles of atoms are there in 3 mol of sodium ethoxide C₂H₅ONa?
3
Find the sum of the oxidation states of both carbon atoms in ethane CH₃–CH₃.
4
Why do C₂H₅OH, NaOH and CH₃COOH differ in properties although all contain an –OH group?