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Aromatic hydrocarbon

An aromatic hydrocarbon (abbreviated as AH) or arene is a hydrocarbon, of which the molecular structure incorporates one or more planar sets of six carbon atoms that are connected by delocalised electrons numbering the same as if they consisted of alternating single and double covalent bonds.

The term “aromatic” was assigned before the physical mechanism determining aromaticity was discovered, and was derived from the fact that many of the compounds have a sweet scent. This sweet scent actually came from impurities in the compounds (which are not actually aromatic in the sense initially described).

The configuration of six carbon atoms in aromatic compounds is known as a benzene ring, after the simplest possible such hydrocarbon, benzene. Aromatic hydrocarbons can be monocyclic or polycyclic.

General properties: 1) display aromaticity; 2) the carbon-hydrogenratio is very large; 3) they burn with a sooty yellow flame because of the high carbon-hydrogen ratio.

 

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Cycloalkanes

Cycloalkanes, also called naphthenes, are types of alkanes which have one or more rings of carbon atoms in the chemical structure of their molecules. Cycloalkanes consist of only carbon (C) and hydrogen (H) atoms and are saturated because there are no multiple C-C bonds to hydrogenate (add more hydrogen to). A general chemical formula for cycloalkanes would be CnH2(n+1-g) where n = number of C atoms and g = number of rings in the molecule.

Cylobutane

Cycloalkanes with a single ring are named analogously to their normal alkane counterpart of the same carbon count: cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc. The larger cycloalkanes, with greater than 20 carbon atoms are typically called cycloparaffins.

Cycloalkanes are classified into small, common, medium, and large cycloalkanes, where cyclopropane and cyclobutane are the small ones, cyclopentane, cyclohexane, cycloheptane are the common ones, cyclooctane through cyclotridecane are the medium ones, and the rest are the larger ones.

Cycloalkanes are similar to alkanes in their general physical properties, but they have higher boiling points, melting points, and densities than alkanes.

Translate the sentences given.

1. Углеводороды могут быть газами (например, метан и пропан), жидкостями (например, гексан и бензол), восками или низкоплавкими твердыми веществами (например, твердый парафин и нафталин).

2. Алканы – это типы органических углеродных соединений, которые имеют только одинарные химические связи в своей структуре.

3. Пропан подвергается реакциям горения подобно другим алканам. При наличии избытка кислорода, пропан горит с образованием воды и двуокиси углерода. Когда кислорода в реакции присутствует недостаточно, происходит неполное сгорание, и пропан горит с образованием воды, окиси углерода, двуокиси углерода и углерода.

4. Ароматические соединения играют очень важную роль в промышленности. Ключевыми ароматическими углеводородами с экономической точки зрения являются бензол, толуол, орто-ксилол (диметилбензол) и пара ксилол.

5. Примерно 35 миллионов тонн ароматических соединений производится каждый год. Их выделяют из сложных смесей, полученных путем переработки нефти или дистилляции каменноугольной смолы (дегтя) (coal tar); они используются для производства ряда важных химических веществ и полимеров, включая стирол (styrene), фенол, анилин (aniline), полиэстр и нейлон.

TEXT 7. ALIPHATIC AND CYCLIC HYDROCARBONS

 

Two overarching chain type categories exist: Open Chain aliphatic compounds and Closed Chain cyclic compounds. Those in which both open chain and cyclic parts are present are normally classed with the latter.

Aliphatic compounds. The aliphatic hydrocarbons are subdivided into three groups (homologous series) according to their state of saturation: paraffins alkanes without any double or triple bonds, olefins alkenes with double bonds, which can be mono-olefins with a single double bond, di-olefins, or di-enes with two, or poly-olefins with more. The third group with a triple bond is named after the name of the shortest member of the homologue series as the acetylenes alkynes. The rest of the group is classed according to the functional groups present.

From another aspect aliphatics can be straight chain or branched chain compounds, and the degree of branching also affects characteristics, like octane number or cetane number in petroleum chemistry.

Aromatic and alicyclic compounds. Cyclic compounds can, again, be saturated or unsaturated. Because of the bonding angle of carbon, the most stable configurations contain six carbon atoms, but while rings with five carbon atoms are also frequent, others are rarer. The cyclic hydrocarbons divide into alicyclics and aromatics (also called arenes).

Of the alicyclic compounds the cycloalkanes do not contain multiple bonds, whilst the cycloalkenes and the cycloalkynes do. Typically this latter type only exists in the form of large rings, called macrocycles. The simplest member of the cycloalkane family is the three-membered cyclopropane.

Aromatic hydrocarbons contain conjugated double bonds. One of the simplest examples of these is benzene, the structure of which was formulated by Kekulé who first proposed the delocalization or resonance principle for explaining its structure. For "conventional" cyclic compounds, aromaticity is conferred by the presence of 4n + 2 delocalized pi electrons, where n is an integer. Particular instability (antiaromaticity) is conferred by the presence of 4n conjugated pi electrons.

The characteristics of the cyclic hydrocarbons are again altered if heteroatoms are present, which can exist as either substituents attached externally to the ring (exocyclic) or as a member of the ring itself (endocyclic). In the case of the latter, the ring is termed a heterocycle. Pyridine and furan are examples of aromatic heterocycles while piperidine and tetrahydrofuran are the corresponding alicyclic heterocycles. The heteroatom of heterocyclic molecules is generally oxygen, sulfur, or nitrogen, with the latter being particularly common in biochemical systems.

Rings can fuse with other rings on an edge to give polycyclic compounds. The purine nucleoside bases are notable polycyclic aromatic heterocycles. Rings can also fuse on a "corner" such that one atom (almost always carbon) has two bonds going to one ring and two to another. Such compounds are termed spiro and are important in a number of natural products.

 

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