How many isomeric alkenes of c4h10 are there

Alkanes with carbons, methane CH 4 , ethane C 2 H 6 , and propane C 3 H 8 , do not exist in isomeric forms because there is only one way to arrange the atoms in each formula so that each carbon atom has four bonds.

However, C 4 H 10 , has more than possible structure. The four carbons can be drawn in a row to form butane or the can branch to form isobutane. Likewise the molecular formula: C 5 H 12 has three possible isomer. The compound at the far left is pentane because it has all five carbon atoms in a continuous chain. The compound in the middle is isopentane; like isobutane, it has a one CH 3 branch off the second carbon atom of the continuous chain.

Although all three have the same molecular formula, they have different properties, including boiling points: pentane, Of the structures show above, butane and pentane are called normal alkanes or straight-chain alkanes , indicating that all contain a single continuous chain of carbon atoms and can be represented by a projection formula whose carbon atoms are in a straight line. The other structures, isobutane, isopentane, and neopentane are called called branched-chain alkanes. As the number of carbons in an akane increases the number of possible isomers also increases as shown in the table below.

Akanes can be represented in many different ways. The figure below shows some of the different ways straight-chain butane can be represented. Note that many of these structures only imply bonding connections and do not indicate any particular geometry. The bottom two structures, referred to as "ball and stick" and "space filling" do show 3D geometry for butane. Because the four-carbon chain in butane may be bent in various ways the groups can rotate freely about the C—C bonds.

However, this rotation does not change the identity of the compound. It is important to realize that bending a chain does not change the identity of the compound; all of the following represent the same compound, butane:. The nomenclature of straight alkanes is based on the number of carbon atoms they contain. The number of carbons are indicated by a prefix and the suffix -ane is added to indicate the molecules is an alkane.

Likewise, the prefix for six is hex so the name for the straight chain isomer of C 6 H 14 is called hexane. The first ten prefixes should be memorized, because these alkane names from the basis for naming many other organic compounds.

Pentane, C 5 H 12 , has three chain isomers. If you think you can find any others, they are simply twisted versions of the ones below.

If in doubt make some models. Draw all of the isomers for C 6 H 14 O that contain a 6 carbon chain and an alcohol -OH functional group. Draw all possible isomers for C 6 H 14 There are five total. Despite their relative inertness, alkanes undergo several important reactions that are discussed in the following section. Combustion The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history.

The practical importance of this reaction cannot be denied, but the massive and uncontrolled chemical changes that take place in combustion make it difficult to deduce mechanistic paths.

Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. No other common reaction involves such a profound and pervasive change, and the mechanism of combustion is so complex that chemists are just beginning to explore and understand some of its elementary features.

Two points concerning this reaction are important: 1. Since all the covalent bonds in the reactant molecules are broken, the quantity of heat evolved in this reaction is related to the strength of these bonds and, of course, the strength of the bonds formed in the products. Precise heats of combustion measurements can therefore provide useful iinformation about the structure of molecules. The stoichiometry of the reactants is important. If insufficient oxygen is supplied some of the products will consist of carbon monoxide, a highly toxic gas.

As noted in the reaction energetics section isomers may have different potential energies , reflecting the bond energies and strain in each. Since isomeric hydrocarbons must give the same mixture of CO 2 and H 2 O on complete combustion, differences in their potential energy will be revealed by their heats of combustion. The diagram on the left below demonstrates how the heat of combustion of isomeric hydrocarbons in this case C 6 H 12 compounds provides information about their thermodynamic stability.

Cyclohexane is clearly the most stable lower potential energy of the four isomers depicted. In small-ring cyclic compounds ring strain can be a major contributor to thermodynamic instability and chemical reactivity. The table on the right lists heat of combustion data for some simple cycloalkanes, and compares the heat derived per CH 2 unit with that of a long chain alkane.

The strain induced by the structural constraint of a small ring is evident. Chemists recognize several kinds of internal molecular strain. Among these, angle strain and eclipsing strain bond orientation are severe in small rings such as cyclopropane and cyclobutane.

A third strain, steric hindrance crowding is common in branched or substituted chains and rings. These strain factors will be discussed in the next chapter. Changes in chemical reactivity as a consequence of angle strain are dramatic in the case of cyclopropane, and are also evident for cyclobutane.

Some examples are shown in the following diagram. The cyclopropane reactions are additions, many of which are initiated by electrophilic attack. Unstrained hydrocarbons require much higher temperatures to effect this kind of bond cleavage, which generally leads to mixtures of lower molecular weight alkanes and alkenes. Such structural degadation or cracking is used by the petrochemical industry to convert the abundant high-molecular weight components of petroleum into the simple hydrocarbons needed as feedstocks and fuels.

Halogenation Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen fluorine, chlorine, bromine or iodine. Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed.

The chlorination of methane, shown below, provides a simple example of this reaction. However, one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products, as shown in the following unbalanced equation.

The relative amounts of the various products depend on the proportion of the two reactants used. In the case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the chief product; whereas, an excess of chlorine favors formation of chloroform and carbon tetrachloride. The following facts must be accomodated by any reasonable mechanism for the halogenation reaction. We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive.

Chlorinations and brominations are normally exothermic. Energy input in the form of heat or light is necessary to initiate these halogenations. If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed. Halogenation reactions may be conducted in either the gaseous or liquid phase. In gas phase chlorinations the presence of oxygen a radical trap inhibits the reaction.

In liquid phase halogenations radical initiators such as peroxides facilitate the reaction. The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. A chain reaction mechanism for the chlorination of methane was described earlier. Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.

To see an animated model of the bromination free radical chain reaction. When alkanes larger than ethane are halogenated, isomeric products are formed.

Thus chlorination of propane gives both 1-chloropropane and 2-chloropropane as mono-chlorinated products. Four constitutionally isomeric dichlorinated products are possible, and five constitutional isomers exist for the trichlorinated propanes.

Can you write structural formulas for the four dichlorinated isomers? The halogenation of propane discloses an interesting feature of these reactions. All the hydrogens in a complex alkane do not exhibit equal reactivity. For example, propane has eight hydrogens, six of them being structurally equivalent primary , and the other two being secondary. This is not what we observe. It should be clear from a review of the two steps that make up the free radical chain reaction for halogenation that the first step hydrogen abstraction is the product determining step.

Once a carbon radical is formed, subsequent bonding to a halogen atom in the second step can only occur at the radical site. Since the H-X product is common to all possible reactions, differences in reactivity can only be attributed to differences in C-H bond dissociation energies. In our previous discussion of bond energy we assumed average values for all bonds of a given kind, but now we see that this is not strictly true.

In the case of carbon-hydrogen bonds, there are significant differences, and the specific dissociation energies energy required to break a bond homolytically for various kinds of C-H bonds have been measured.

These values are given in the following table. By this reasoning we would expect benzylic and allylic sites to be exceptionally reactive in free radical halogenation, as experiments have shown. The methyl group of toluene, C 6 H 5 CH 3 , is readily chlorinated or brominated in the presence of free radical initiators usually peroxides , and ethylbenzene is similarly chlorinated at the benzylic location exclusively. In one isomer, both methyl groups are on the same side of the double bond cis butene and in the other, the methyl groups are on opposite sides of the double bond trans butene :.

The two isomers clearly have the same structural framework but they differ in the arrangement of this framework in space — hence the designation stereoisomers. They owe their separate existence to the fact that the double bond is rigid and the parts of the molecule are not free to rotate with respect to each other about this bond. Therefore the isomers do not interconvert without breaking the double bond, and they exist as different compounds, each with its own chemical and physical properties.

Ball-and-stick models of cis- and trans butene are shown below, and the rigidity of the double bond is simulated in the model by a pair of stiff springs or bent sticks connecting the two carbons of the double bond. It should be clear to you that there will be no cis-trans isomers of alkenes in which one end of the double bond carries identical groups.

Thus we don not expect there to be cis-trans isomers of 1-butene or 2-methylpropene, and. Ring formation also confers rigidity on molecular structure such that rotation about the ring bonds is prevented. As a result, stereoisomerism of the cis-trans type is possible. For example, 1,2-dimethylcyclopropane exists in two forms that differ in the arrangement of the two methyl groups with respect to the ring. Ball-and-stick models of cis and trans isomers of 1,2-dimethylcyclopropane.

In the cis isomer, the methyl groups both are situated above or below the plane of the ring and in the trans isomer they are situated one above and one below, as shown in the figure. Interconversion of these isomers does not occur without breaking one or more chemical bonds. Stereoisomers that do not interconvert rapidly under normal conditions, and therefore are stable enough to be separated, specifically are called configurational isomers.

Thus cis — and trans butene are configurational isomers, as are cis — and trans -1,2-dimethylcyclopropane. The terms cis-trans isomerism or geometric isomerism commonly are used to describe configurational isomerism in compounds with double bonds and rings.

When referring to the configuration of a particular isomer, we mean to specify its geometry. For instance, the isomer of 1,2-dichloroethene shown below has the trans configuration; the isomer of 1,3-dichlorocyclobutane has the cis configuration:. Cis-trans isomerism is encountered very frequently. By one convention, the configuration of a complex alkene is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is trans ethylmethylheptene, despite the fact that two identical groups are cis with respect to each other, because the longest continuous chain is trans as it passes through the double bond:.

Notice that cis-trans isomerism is not possible at a carbon-carbon triple bond, as for 2-butyne, because the bonding arrangement at the triply bonded carbons is linear:. Many compounds have more than one double bond and each may have the potential for the cis or trans arrangement. For example, 2,4-hexadiene has three different configurations, which are designated as trans-trans, cis-cis, and trans-cis.

Because the two ends of this molecule are identically substituted, the trans-cis becomes identical with cis-trans:. For example, it is very tempting to draw butene as. If you write it like this, you will almost certainly miss the fact that there are geometric isomers.

In other words, use the format shown in the last diagrams above. You obviously need to have restricted rotation somewhere in the molecule. Compounds containing a carbon-carbon double bond have this restricted rotation as do compounds with multiple groups attached to a ring, so you need to consider the possibility of geometric isomers. To get from one to the other, all you would have to do is to turn the whole model over. So there must be two different groups on the left-hand carbon and two different groups on the right-hand one.