Ring Compounds, Benzene and Catenation വലയസംയുക്തങ്ങൾ
So far every carbon chain had two loose ends. But a chain can also bend round and bite its own tail, closing into a ring. Moth balls (paattagulika) in a cupboard, the smell of some paints and glues, and many medicines all contain rings of carbon atoms. On this page you will close a chain into a ring with your own hands, build benzene the way Kekulé imagined it, and find out why carbon makes more compounds than all the other elements put together.
1. Carbon chains that close into rings (Fig 8.7)
കാർബൺ ആറ്റങ്ങളുടെ ഏക ബന്ധനമുള്ളതും, ദ്വിബന്ധനമുള്ളതും, ത്രിബന്ധനമുള്ളതുമായ ധാരാളം ഓപ്പൺ ചെയിൻ (open chain) സംയുക്തങ്ങളെ പരിചയപ്പെട്ടുവല്ലോ. You have met many open-chain ഓപ്പൺ ചെയിൻ compounds with single, double and triple bonds between carbon atoms. ചുവടെ നൽകിയിരിക്കുന്ന ഏതാനും കാർബൺ സംയുക്തങ്ങളുടെ ഘടന ശ്രദ്ധിക്കൂ. Now look at the structures of these carbon compounds.
Q (page 164). ഈ കാർബൺ സംയുക്തങ്ങളുടെ ഘടനയിലുള്ള പ്രത്യേകത എന്ത്? What is special about the structures of these carbon compounds?
കാർബൺ ആറ്റങ്ങൾക്ക് പരസ്പരം സംയോജിച്ച് വലയരൂപത്തിലുള്ള സംയുക്തങ്ങളുണ്ടാക്കാനുമുള്ള കഴിവുണ്ട്. വലയ ഹൈഡ്രോകാർബൺ സംയുക്തങ്ങളെ ആലിസൈക്ലിക് സംയുക്തങ്ങൾ, അരോമാറ്റിക് സംയുക്തങ്ങൾ എന്നിങ്ങനെ രണ്ടായി തരംതിരിക്കാം. Carbon atoms can also join one another to form compounds in the shape of a ring. Ring hydrocarbons are classified into two groups: alicyclic compounds ആലിസൈക്ലിക് സംയുക്തങ്ങൾ and aromatic compounds അരോമാറ്റിക് ഹൈഡ്രോകാർബണുകൾ.
2. Alicyclic compounds (Fig 8.8) ആലിസൈക്ലിക് സംയുക്തങ്ങൾ
ആൽക്കെയ്ൻ, ആൽക്കീൻ, ആൽക്കൈൻ എന്നീ ഓപ്പൺ ചെയിൻ ഹൈഡ്രോകാർബണുകളോട് സമാനമായ വലയഹൈഡ്രോകാർബണുകളാണ് ഇവ. These are ring hydrocarbons that are similar to the open-chain alkanes, alkenes and alkynes. A ring with only single bonds is like an alkane; a ring with a double bond is like an alkene.
Cyclohexene and cyclopentane are not in Fig 8.8, but you need them for "Let us assess" and the extended activities. The drawing throws an error if any carbon does not have exactly 4 bonds, and the molecular formula is counted from the drawing itself.
Fig 8.8 (page 165). ചില ആലിസൈക്ലിക് ഹൈഡ്രോകാർബണുകളുടെ ഘടന, IUPAC നാമം എന്നിവ നൽകിയിരിക്കുന്നു (ചിത്രം 8.8). ഇവയുടെ തന്മാത്രാസൂത്രം എഴുതുക. The structures and IUPAC names of some alicyclic hydrocarbons are given in Fig 8.8. Write their molecular formulas. Count the C and H atoms in the gallery above. (Type C4H8 – digits are fine.)
Q (page 165). ആലിസൈക്ലിക് ഹൈഡ്രോകാർബണുകൾക്ക് പേര് നൽകിയിരിക്കുന്നവിധം കണ്ടെത്തൂ. Find out how the alicyclic hydrocarbons have been named.
Just write the word "cyclo" as a prefix മുൻപ്രത്യയം before the IUPAC name of the hydrocarbon: cyclo + propane → cyclopropane, cyclo + hexane → cyclohexane, cyclo + butene → cyclobutene.
Closing a chain into a ring uses one more C–C bond, so the two end carbons each need one H less. That is why cyclobutane is C₄H₈, not C₄H₁₀, and cyclohexane C₆H₁₂: a ring with only single bonds fits CₙH₂ₙ – the same formula as an alkene. So a formula like C₅H₁₀ can be pentene or cyclopentane. When a question just says "C₅H₁₀ – which family?", the textbook answer is alkene.
3. Close the ring yourself: butene → cyclobutane
Q (page 166). ഒരു ഹൈഡ്രോകാർബണിന്റെ ഘടനാവാക്യം ചുവടെ നൽകിയിരിക്കുന്നു. The structural formula ഘടനാവാക്യം of a hydrocarbon is given: CH₂ = CH – CH₂ – CH₃
a. ഈ സംയുക്തത്തിന്റെ തന്മാത്രാസൂത്രം എഴുതുക. Write the molecular formula of this compound.
b. ഇതേ തന്മാത്രാസൂത്രമുള്ള ഒരു വലയ സംയുക്തത്തിന്റെ ഘടന വരയ്ക്കുക. Draw the structure of a ring compound with the same molecular formula. Step through the animation – it rearranges the same 4 C and 8 H atoms.
The animation is a paper exercise for counting atoms and bonds – it is not how butene is turned into cyclobutane in a real laboratory.
4. Aromatic hydrocarbons and benzene അരോമാറ്റിക് ഹൈഡ്രോകാർബണുകൾ
തനതായ ഗന്ധമുള്ള വലയസംയുക്തങ്ങളാണ് അരോമാറ്റിക് സംയുക്തങ്ങൾ. വ്യാവസായിക പ്രാധാന്യമുള്ള ഒരു അരോമാറ്റിക് സംയുക്തമാണ് ബെൻസീൻ (Benzene). Aromatic compounds are ring compounds with a characteristic smell ("aroma"). Benzene ബെൻസീൻ is an aromatic compound of great industrial importance. Look at its structure (Fig 8.9) – or build it below.
Q (page 165). ബെൻസീനിന്റെ തന്മാത്രാസൂത്രം എഴുതിനോക്കൂ. Write the molecular formula of benzene.
Benzene has been produced industrially since the nineteenth century. It was discovered by Michael Faraday in 1825 through his experiments. For many years the chemists of the time could not explain its structure.
August Kekulé (1829–1896), who studied the structures of chemical compounds, tried to work it out. It is said that a dream led Kekulé to the structure of benzene (the story goes that he dreamt of a snake biting its own tail). In 1865 he published his research paper on the structure of benzene, the discovery that made him famous worldwide. He gave benzene a hexagonal (six-sided) structure in which six carbon atoms and six hydrogen atoms are held together with alternating double bonds.
The double bonds can be drawn in two ways (shifted by one position). Kekulé suggested that benzene keeps switching between them. Today we know all six C–C bonds in benzene are exactly the same – halfway between single and double – so chemists often draw a hexagon with a circle inside. In this class, draw the Kekulé structure with three alternating double bonds, as in Fig 8.9.
Naphthalene – moth balls നാഫ്തലീൻ · പാറ്റാഗുളിക
പ്രത്യേക ഗന്ധമുള്ളതും വെളുത്ത ക്രിസ്റ്റലാകൃതിയുള്ളതുമായ മറ്റൊരു അരോമാറ്റിക് ഹൈഡ്രോകാർബണാണ് നാഫ്തലീൻ. ഇതിന്റെ ഘടനയിൽ രണ്ട് ബെൻസീൻ വലയങ്ങൾ ഒരുമിച്ച് ചേർന്നിരിക്കുന്നു. പാറ്റാഗുളിക (moth ball) യിലെ പ്രധാന ഘടകമാണിത്. Naphthalene is another aromatic hydrocarbon. It forms white crystals with a characteristic smell. In its structure two benzene rings are joined together (they share one side). It is the main component of moth balls പാറ്റാഗുളിക (Fig 8.10). Choose "naphthalene" in the ring gallery above to count its atoms.
Q (page 165). നാഫ്തലീനിന്റെ തന്മാത്രാസൂത്രം എഴുതുക. Write the molecular formula of naphthalene.
5. Compare compound A and compound B
Q (page 166). രണ്ട് ഓർഗാനിക് സംയുക്തങ്ങളുടെ ഘടന ചുവടെ നൽകിയിരിക്കുന്നു. ഇവ താരതമ്യം ചെയ്യുക. The structures of two organic compounds are given. Compare them.
Different: A (benzene, C₆H₆) has alternating double and single bonds, one H on each carbon, is unsaturated and aromatic (characteristic smell). B (cyclohexane, C₆H₁₂) has only single bonds, two H on each carbon, is saturated and alicyclic (a ring version of hexane). B has 6 more hydrogen atoms than A.
6. Catenation – why there are so many carbon compounds കാറ്റിനേഷൻ
Catenation is the ability of atoms of the same element to join with each other. Carbon has the greatest ability for catenation. This is one reason why so many carbon compounds are found in nature.
Try 4 carbons: the straight chain and the branched chain have the same formula C₄H₁₀ but different structures (go further: such compounds are called isomers). The ring is C₄H₈. More shapes → more compounds.
Q (page 166). കാർബൺ സംയുക്തങ്ങളുടെ എണ്ണം വളരെ കൂടുതലാണെന്ന് മനസ്സിലായല്ലോ. ഇതിനുള്ള കാരണങ്ങൾ ലിസ്റ്റ് ചെയ്യുക. You have understood that the number of carbon compounds is very large. List the reasons for this.
First a quick check: which of these is not a reason?
- Catenation കാറ്റിനേഷൻ: carbon atoms join one another to form very long chains – the strongest catenation of all elements.
- Valency 4: each carbon can bond to four other atoms, so chains can grow in many directions.
- Carbon forms single, double and triple bonds (alkanes, alkenes, alkynes).
- Carbon chains can be straight, branched or closed into rings (open-chain, alicyclic and aromatic compounds).
- Carbon also bonds strongly with other elements – hydrogen, oxygen, nitrogen, sulphur, chlorine and other halogens – giving alcohols, acids, sugars, proteins and many more.
- (Go further) The same molecular formula can have different structures – isomers – such as butene and cyclobutane, both C₄H₈.
- Forgetting the cyclo prefix: a ring of 4 carbons with single bonds is cyclobutane, not butane (butane is C₄H₁₀, cyclobutane C₄H₈).
- Thinking benzene and cyclohexane have the same formula ✗ – benzene is C₆H₆, cyclohexane C₆H₁₂.
- Drawing benzene with 2 H on each carbon or with all double bonds ✗ – each carbon has 1 H, one double and one single ring bond: 4 bonds.
- Writing naphthalene as C₁₂H₁₂ ✗ – the two rings share two carbons: C₁₀H₈.
- "Catenation = carbon joining with hydrogen" ✗ – it is atoms of the same element joining each other.
- 📄 Cycloalkane Wikipedia
Ring hydrocarbons like cyclopropane and cyclohexane. - 📄 Benzene Wikipedia
The six-carbon aromatic ring. - 📄 August Kekulé Wikipedia
Proposed the ring structure of benzene in 1865. - 📄 Naphthalene Wikipedia
Two fused benzene rings – moth balls.
7. Check yourself
- Ring (cyclic) compounds വലയസംയുക്തങ്ങൾ: carbon atoms joined in a closed ring (Fig 8.7). Two kinds: alicyclic and aromatic.
- Alicyclic: rings like the open-chain alkanes/alkenes – cyclopropane C₃H₆, cyclobutane C₄H₈, cyclobutene C₄H₆, cyclopentene C₅H₈, cyclohexane C₆H₁₂ (Fig 8.8). Name = cyclo + IUPAC name.
- Aromatic: ring compounds with a characteristic smell – benzene C₆H₆ (Faraday 1825; Kekulé 1865: hexagon with alternating double bonds) and naphthalene C₁₀H₈ (two fused benzene rings, white crystals, moth balls).
- CH₂=CH–CH₂–CH₃ is C₄H₈; the ring with the same formula is cyclobutane. Benzene (C₆H₆, unsaturated, aromatic) ≠ cyclohexane (C₆H₁₂, saturated, alicyclic).
- Catenation: atoms of the same element joining each other; carbon has it most. With valency 4, multiple bonds, chains/branches/rings and bonds to other elements, it explains the huge number of carbon compounds.