bonding in organic compounds




Why is carbon unique? What accounts for the apparently limitless number of carbon compounds that can be prepared? The answer is that bonds between carbon atoms are stable, allowing chains of carbon atoms to be formed, with each carbon atom of a chain being capable of joining to other atoms such as hydrogen, oxygen, sulfur, nitrogen, and the halogens. Neighboring atoms in the periodic table, such as boron, silicon, sulfur, and phosphorus, can also bond to themselves to form chains in the elemental state, but the resulting compounds are generally quite unstable and highly reactive when atoms of hydrogen or halogen, for example, are attached to them. The elements at the right or left of the periodic table do not form chains at all-their electronattracting or electron-repelling properties are too great.

The forces that hold atoms and groups of atoms together are the electrostatic forces of attraction between positively charged nuclei and negatively charged electrons on different atoms. We usually recognize two kinds of binding. The first is the familiar ionic bond that holds a crystal of sodium chloride together. Each Na@ in the crystal feels a force of attraction to each ClO, the force decreasing as the distance increases. (Repulsion between ions of the same sign of charge is also present, of course, but the stable crystal arrangement has more attraction than repulsion.) Thus, you cannot identify a sodium chloride pair as being a molecule of sodium chloride. Similarly, in an aqueous solution of sodium chloride, each sodium ion and chloride ion move in the resultant electric field of all the other ions in the solution. Sodium chloride, like other salts, can be vaporized at high temperatures. The boiling point of sodium chloride is 1400°.' For sodium chloride vapor, you can at last speak of sodium chloride molecules which, in fact, are pairs of ions, Na@ClO. Enormous energy is required to vaporize the salt because in the vapor state each ion interacts with just one partner instead of many.

The second kind of bonding referred to above results from the simultaneous interaction of a pair of electrons (or, less frequently, just one electron) with two nuclei, and is called the covalent bond. Whereas metallic sodium reacts with chlorine by completely transferring an electron to it to form Na@ and clO, the elements toward the middle of the rows of the periodic table tend to react with each other by sharing electrons.

Transfer of an electron from a sodium atom to a chlorine atom produces two ions, each of which now possesses an octet of electrons. This means of achieving an octet of electrons is not open to an element such as carbon, which has two electrons in a filled inner K shell and four valence electrons in the outer L shell. A quadrinegative ion C4@ with an octet of electrons in the valence shell would have an enormous concentration of charge and be of very high energy, Similarly, the quadripositive ion C4@, which would have a filled K shell like helium, would be equally unstable. Carbon (and to a great extent boron, nitrogen, oxygen, and the halogens) completes its valence-shell octet by sharing electrons with other atoms.

In compounds with shared electron bonds (or covalent bonds) such as methane (CH4,) or tetrafluoromethane (CF4,), carbon has its valence shell filled, as shown in these Lewis structures:

bonding in organic compounds

For convenience, these molecules are usually written with each bonding pair of electrons represented by a dash:

bonding in organic compounds

 



Frequently Asked Questions

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Ans: Why is carbon unique? What accounts for the apparently limitless number of carbon compounds that can be prepared? The answer is that bonds between carbon atoms are stable, allowing chains of carbon atoms to be formed, with each carbon atom of a chain being capable of joining to other atoms such as hydrogen, oxygen, sulfur, nitrogen, and the halogens. view more..
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Ans: Lithium and beryllium are able to form positive ions by loss of one or two electrons, respectively. Boron is in an intermediate position and its somewhat unusual bonding properties are considered later in the book (Section 19.5). view more..
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Ans: Hydrocarbons are compounds that contain only carbon and hydrogen. We shall consider in this chapter the four simplest known hydrocarbonsthose with the lowest molecular weights-and we shall see that they represent three classes of compounds: the alkanes, in which each carbon atom has four single bonds; the alkenes, in which two carbon atoms are joined by a double bond (two electron pairs); and the alkynes, in which two carbon atoms are joined by a triple bond (three electron pairs). view more..
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Ans: In organic chemistry, the word structure has a specific meaning; It designates the order in which the atoms are joined to each other. A structure does not necessarily specify the exact shape of a molecule because rotation about single bonds could lead, even for a molecule as simple as ethane, to an infinite number of different arrangements of the atoms in space. view more..
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Ans: The two simplest unsaturated compounds (those containing a multiple bond) are ethene (CH,=CH,) and ethyne (HCzCH). The generally lower stability of multiply bonded compounds arises from the restriction that only one electron pair can occupy a given orbital view more..
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Ans: In the previous two chapters we have studied in some detail the properties of the two simplest saturated hydrocarbons, methane and ethane, and have shown how their simple derivatives are named using the rules of the International Union of Pure and Applied Chemistry (IUPAC rules). view more..
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Ans: The series of continuous-chain alkanes, CH,(CH,),-,CH, , shows a remarkably smooth gradation of physical properties (see Table 3.3 and Figure 3-2). As you go up the series, each additional CH, group contributes a fairly constant increment to the boiling point and density and, to a lesser extent, to the melting point. view more..
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Ans: As a class, alkanes are singularly unreactive. The name saturated hydrocarbon (or " paraffin," which literally means " little affinity " [L. par(um), little, + afins, affinity1)arises because their chemical affinity for most common reagents may be regarded as saturated or satisfied. view more..
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Ans: An important and interesting group of hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple unsubstituted cycloalkanes of the formula (CH,), make up a particularly important homologous series in which the chemical properties change in a much more striking way than do the properties of the open-chain hydrocarbons, CH3(CH2),-,CH3. view more..
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Ans: In the early days of organic chemistry, when it was found that the alkenes, but not the alkanes, readily undergo addition reactions with substances such as halogens, hydrogen halides, sulfuric acid, and oxidizing agents, the chemical affinity of alkanes was said to be " saturated" while that of the alkenes was said to be " unsaturated." Now, even though we recognize that no chemical entity (even the noble gases such as helium and xenon) can surely be classified as saturated, the description of alkanes and alkenes as saturated and unsaturated is still commonly used. view more..
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Ans: In the homologous series of alkanes, isomerism first appears at the C, level, two compounds of formula C4H,, being known. These are structural isomers: view more..
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Ans: By convention, the configuration of complex alkenes is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is 4-ethyl-3-methyl-trans-3- heptene, despite the fact that two identical groups are cis with respect to each view more..
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Ans: We have previously examined briefly two addition reactions of ethene, the first member of the homologous series of alkenes. These were addition of hydrogen, catalyzed by surfaces of finely divided metals such as nickel, and the addition of bromine. view more..




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