>> Key Terms (indicated in blue within the text below):

-glycosidic alcohol aldehyde alkane alkene alkyne amine aromatic benzene -glycosidic boat conformation carbohydrate carbonyl carboxylic acid cellulose chair conformation cis combustion

condensation coupling reaction cyclic compound cyclization disaccharide empirical formula ether fuel value functional group geometric isomer halide hemiacetal homologous series hydrocarbon hydrolysis hydroxyl interstices iso

ketone meta monosaccharide n-alkane organic compound ortho ose para polysaccharide primary amine secondary amine starch straight-chain structural isomer sugar tertiary amine tertiary carbon trans

Organic compounds are based on carbon. If the compound contains only carbon and hydrogen, it is a hydrocarbon. However, many organic compounds also contain oxygen or nitrogen atoms in addition to the hydrogen and carbon.

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There are many ways these atoms can be arranged. Two compounds with the same chemical formula (number and type of atoms) but different bonding patterns (Lewis structures) are called structural isomers. The chemical and physical properties of the compounds depend on the arrangement. Chemical properties, in particular, are often determined by a small group of atoms with a particular arrangement. These groups of atoms are the part of the organic molecule called a functional group.

These functional groups are generally arrangements of atoms that differ from the basic model of carbons singly bonded to other carbons or hydrogens (alkanes). Functional groups of hydrocarbons include alkenes, where there is a carbon–carbon double bond and alkynes, where there is a carbon–carbon triple bond. If the organic compound contains a six-membered ring of carbons with three alternating double bonds, it is an aromatic. Aromatics have a resonance structure, where the double bonds are shifted one ring position. Since the true structure is a mixture of all resonance structures, aromatics actually have a double-bond ring within the single-bonded ring and the ring geometry is planar.

Functional groups involving oxygen include alcohols, where an OH group (hydroxyl group) is bonded to a carbon, and ethers, where there is a carbon–oxygen–carbon bond. A molecule that has a carbon bonded to both a hydroxyl group and the oxygen of an ether is called a hemiacetal. There are three types of carbonyls, where a carbon is doubly bonded to an oxygen. If the carbonyl carbon is also bonded to a hydrogen, it is an aldehyde. If the carbonyl carbon is also bonded to a hydroxyl group, it is a carboxylic acid, and if the carbon is bonded to two other carbons, it is a ketone.

Functional groups involving nitrogen are amines. If the nitrogen is bonded to only one carbon, it is a primary amine. If the nitrogen is bonded to two carbons, it is a secondary amine. If the nitrogen is bonded to three carbons, it is a tertiary amine.

Carbons may also bond singly to a halogen instead of a hydrogen. These are halide functional groups.

For alkanes the simplest arrangement of carbons is for all but the end carbons to be bonded to two other carbons and two hydrogens. The end carbons are bonded to one other carbon and three hydrogens. These are called straight-chain or normal alkanes (n-alkanes). These saturated hydrocarbons have the formula C_nH_{2n + 2}. Although called "straight chain," each carbon actually has a tetrahedral orientation. Thus the angle between carbons is 109.5° and the chain actually has a zig-zag shape.

If a carbon is bonded to three other carbons, the carbons are no longer in a "straight chain." This can be designated by the prefix iso. If a carbon is not bonded to any hydrogens, it is called a tertiary carbon.

Another arrangement is for the carbons to form a ring, so there are no end carbons. The formula of these cyclic compounds is C_nH_2n. The tetrahedral angles between the carbons create puckered, rather than flat, rings. A six-membered ring is just the right size for the carbons to maintain the angle of a tetrahedral. This six-carbon ring can be oriented in two ways. If opposite ends both point up, it is called the boat conformation. If one end points up and the opposite points down, the conformation is called a chair. The molecule can move back and forth between these two conformations, but the chair orientation is preferred, since it is slightly lower in energy. See Figure 12.00 in the textbook.

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The naming of organic compounds must not only detail the number and type of atoms, but the way the atoms are bonded to each other. Names are based on the functional group and the carbon backbone, the longest line of carbons bonded to each other.

In naming organic compounds, the root word refers to the number of carbons in the longest continuous chain of carbons. These can be found in Tables 11.1 and 12.1 as straight-chain alkanes. (All carbons are in one chain.) The suffix of the name is determined by the functional group. Alkenes end in ene and alkynes, in yne.

Alcohols have an ol suffix. Aldehydes end in al. Ketones end in one. Carboxylic acids have an ic acid ending. Ethers and amines use the entire word in their names. If it is necessary to describe the position of the functional group, each carbon of the backbone is numbered, starting at the end closest to the functional group. The number of the first carbon associated with the group, followed by a dash, precedes the name.

Branches of smaller carbon chains off the main chain are also denoted by prefixes and named with the same root words as the carbon backbone, but with a yl ending. The prefix also designates to which carbon the branch is attached. It does this by numbering the carbons in the backbone. The name of the branch is preceded by the number of carbon it is attached to, followed by a dash, then the name signifying the number of carbons in the branch. The carbon backbone is always numbered in such a way as to keep as low as possible the numbers used in the prefixes.

If there is more than one branch of the same type, the numbers of the carbons they are attached to are grouped together (separated by commas) and a prefix is added before the name of the branch to signify how many of these branches are present. Prefixes used for this purpose are the same as those used to name binary inorganic compounds in Chapter 4.

Halogens attached to carbons are numbered in the same way as the branches. The root name of the halogen, with an o ending, is used to name the halogen.

One aromatic ring with one hydrogen bonded to each carbon is called benzene (C₆H₆). If two of those hydrogens are replaced by other groups, the position of these groups is commonly described by a prefix. If the two groups are next to each other, the prefix used is ortho. If the groups are on opposite sides of the ring, the prefix is para. If the groups are separated by one hydrogen, the prefix is meta.

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Prefixes and suffixes are strung together as necessary to completely describe the compound.

Alkenes have not only structural isomers, where atoms are bonded differently, but also geometric isomers. In a geometric isomer, atoms are bonded to the same atoms but arranged differently in space. Geometric isomers are best described by considering the same group on each of the two carbons connected by a double bond. If the groups are on the same side of the double bond, it is called a cis isomer. If the groups are on the opposite sides of the double bond, it is called a trans isomer. Cis and trans can be used as prefixes to designate the type of geometric isomer.

Carbohydrates or sugars are another type of organic molecule, containing carbon, oxygen, and hydrogen. The name carbohydrate comes from the ratio of hydrogen to oxygen that is the same in these compounds as in water. In reality, the hydrogen and oxygen atoms are not arranged as they are in water, but primarily as alcohols. Ketones, aldehydes, and ether functional groups might also be present. Although sugars are not named systematically, the suffix ose indicates that the substance is a sugar. Some simple sugars can be described (not named) based on the number of carbons (named with the same root as all organic molecules) and the ose suffix and a prefix of aldo if the sugar contains an aldehyde and keto if the sugar contains a ketone. These types of sugars can react with themselves, creating a hemiacetal from an alcohol group and the carbonyl group in the sugar. This reaction is called cyclization because a ring is also formed.

Two simple sugars, called monosaccharides, in the ring form can combine to create a disaccharide. If more monosaccharides are added, this combination of many simple sugars is called a polysaccharide. The coupling reaction combines an alcohol group on the hemiacetal carbon with a hydroxyl group of the other monosaccharide, which creates an ether link and produces water as a product. This reaction can also be described as a condensation reaction because water formed as another product. The opposite of a condensation reaction, where water acts as a reactant, is hydrolysis.

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The geometry of the ether linkage of the polysaccharide affects its chemical properties. To see the difference in geometry, the ring form of the monosaccharide is oriented so that the ether (which is part of the ring) is in the back and the CH₂OH group is oriented in the back and on top of the ring. The hydroxyl group of the hemiacetal carbon may end up either above or below the ring. If it is above the ring, an -glycosidic bond is created when that hydroxyl group couples with another sugar. If the hydroxyl group is below the ring, the ether link to another sugar is a -glycosidic link. This is important because a series of -glycosidic links forms a starch, which is digestible by humans. A polysaccharide consisting of -glycosidic links forms cellulose, which is not.

The combustion of organic compounds can be used to determine their simplest or empirical formula. When an organic compound reacts with excess oxygen (combustion), all the carbon is converted to carbon dioxide and all the hydrogen is converted to water. If these products are collected, the mass of carbon dioxide can be used to determine the amount of carbon that was in the compound and the mass of water used to determine the amount of hydrogen. The amount of oxygen will be the difference between the mass of the original and the mass of the carbon and hydrogen. The empirical formula is the simplest mole ratio of carbon, hydrogen, and oxygen. If the molar mass is known, the molecular formula can be determined. (Empirical and molecular formulas were determined in a similar manner in Chapter 4.)

The combustion of organic molecules is often used as fuel. Fuel values (J/g fuel) are higher for hydrocarbons than for organic molecules containing oxygen. That is because oxygen atoms have a high mass-to-bond ratio. (A more detailed discussion of fuel values is in Chapter 11.) Hydrogen has the highest fuel value of any molecule. It also has the advantage of producing only water as a combustion product. Unfortunately, its low density makes it very difficult to store. One storage possibility is to use certain metals that can absorb hydrogen into the spaces between the metal atoms, interstices. This allows hydrogen to be stored efficiently and as a solid.

The functional groups significantly affect physical properties like melting and boiling points. As discussed in Chapter 9, these properties are determined by intermolecular forces. For example, alcohols, where a hydrogen is directly bonded to an oxygen, will hydrogen-bond and have relatively high melting and boiling points. Carbonyls and ethers have dipoles. Alkanes are held together by London forces, so larger molecules (those with more carbons) will have higher melting and boiling points. Although London forces also predominate in alkenes and alkynes, these do not pack as well as alkanes and therefore have lower melting and boiling points than an alkane with the same number of carbons.

A group of compounds that differ by only one carbon (or CH₂) group is called a homologous series. The physical and chemical properties of such a series change in a predictable manner. For example, the melting points of a homologous series of n-alkanes increase with each additional carbon.