Substitution reactions happen in which hydrogen atoms in the methane are replaced one at a time by chlorine atoms. You end up with a mixture of chloromethane, dichloromethane, trichloromethane and tetrachloromethane.
The original mixture of a colorless and a green gas would produce steamy fumes of hydrogen chloride and a mist of organic liquids. All of the organic products are liquid at room temperature with the exception of the chloromethane which is a gas. If you were using bromine, you could either mix methane with bromine vapor , or bubble the methane through liquid bromine - in either case, exposed to UV light.
The original mixture of gases would, of course, be red-brown rather than green. One would not choose to use these reactions as a means of preparing these organic compounds in the lab because the mixture of products would be too tedious to separate. The mechanisms for the reactions are explained on separate pages. You would again get a mixture of substitution products, but it is worth just looking briefly at what happens if only one of the hydrogen atoms gets substituted monosubstitution - just to show that things aren't always as straightforward as they seem!
For example, with propane, you could get one of two isomers:. If chance was the only factor, you would expect to get three times as much of the isomer with the chlorine on the end. There are 6 hydrogens that could get replaced on the end carbon atoms compared with only 2 in the middle.
In fact, you get about the same amount of each of the two isomers. If you use bromine instead of chlorine, the great majority of the product is where the bromine is attached to the center carbon atom.
The reactions of the cycloalkanes are generally just the same as the alkanes, with the exception of the very small ones - particularly cyclopropane.
In the presence of UV light, cyclopropane will undergo substitution reactions with chlorine or bromine just like a non-cyclic alkane. However, it also has the ability to react in the dark. In the absence of UV light, cyclopropane can undergo addition reactions in which the ring is broken.
For example, with bromine, cyclopropane gives 1,3-dibromopropane. This can still happen in the presence of light - but you will get substitution reactions as well. The ring is broken because cyclopropane suffers badly from ring strain.
The overlap between the atomic orbitals in forming the carbon-carbon bonds is less good than it is normally, and there is considerable repulsion between the bonding pairs. Alkanes with five or more carbons are liquids and are found as common components of petroleum also called crude oil.
Many cycloalkanes are also found in petroleum products as well including, gasoline, kerosene, diesel, motor oil and many other heavy oils. Natural gas also contains trace levels of nitrogen, carbon dioxide CO 2 , and hydrogen sulfide H 2 S.
Components of petroleum are separated by size using a technique called fractional distillation. The resulting samples include gasoline with alkanes ranging from five to ten carbons in length and kerosene with a mixture of alkanes ranging in carbon length from ten to seventeen.
Alkanes with longer carbon chains are found in diesel fuel, fuel oil, petroleum jelly, paraffin wax, motor oils, and the very highest chain length are used in asphalt. It is estimated that the world uses about 95 million barrels of oil each day! The Environmental Protection Agency estimates that each barrel of oil produces 0.
Note that a metric ton is equivalent to 1, kg. That means that 40,, metric tons of CO 2 or 40,,, kg of CO 2 are released into the atmosphere each day just from oil consumption! That is almost 15 billion metric tons of CO 2 per year! This calculation only includes the consumption of crude oil and not other common fuel sources, such as natural gas, coal, wood, and renewable energy sources like ethanol and biodiesel. In Oregon, it is estimated that during the winter months that it take about 40 BTU per hour per square foot to heat an average, well-insulated home.
If you owned a 2, square foot home, how many BTUs would you require to heat your own for 1 month? If you had the choice of heating your house with natural gas, coal lignite , or wood, how many kg of each of these fuels would be required to heat your home for one month?
How much CO2 would be produced by each fuel each winter month? In the presence of heat or light, alkanes can react with halogens to form alkyl halides or haloalkanes. This type of reaction is called a substitution reaction , because the halogen atom is taking the place of or substituting for one of the hydrogen atoms on the alkane structure.
It should be noted that not all of the halogens react in the same way with alkanes. But the reaction goes so fast, that this is the result:. In the presence of a flame, the reactions are rather like the fluorine one — producing a mixture of carbon and the hydrogen halide. The violence of the reaction drops considerably as you go from fluorine to chlorine to bromine. The interesting reactions happen in the presence of ultra-violet light sunlight will do.
These are photochemical reactions that happen at room temperature. In the substitution reaction, a hydrogen atom in the methane is replaced by a chlorine atom. This can happen multiple times, until all the hydrogens are replaced. Ultimately, the longer the reaction proceeds, the more hydrogens in the alkane are replaced.
The original mixture of a colorless gas CH 4 and a green gas Cl 2 would produce steamy fumes of hydrogen chloride HCl and a mist of organic liquids mixture of the chlorinated methane. All of the organic products are liquid at room temperature with the exception of the chloromethane CH 3 Cl which is a gas. This substitution reaction is an example of a radical reaction, where only one electron is transferred at a time. The heat or light, initiates the reaction by breaking the bond between the two Cl atoms in the chloride ion.
This forms two radicals. A radical is an atom, molecule, or ion that has unpaired valence electrons. Thus, they are very unstable and reactive. In the diagram below, the first step of the halogenation reaction is shown below. This is termed initiation. Once the radical is initiated, it will attack the alkane, in this case methane CH 4 , and create a new carbon radical.
This stage of the reaction is termed propagation , as one radical species creates, or propagates, another radical. The final stage of a radical reaction is the termination reaction which quenches the radical species present. For the methane — chlorine reaction this is the formation of the chloromethane CH 3 Cl. For example, with propane, you could get one of two isomers:. If chance was the only factor, you would expect to get three times as much of the isomer with the chlorine on the end.
There are 6 hydrogens that could get replaced on the end carbon atoms compared with only 2 in the middle. In fact, you get about the same amount of each of the two isomers. If you use bromine instead of chlorine, the great majority of the product is where the bromine is attached to the center carbon atom.
Why does this happen? It has to do with the stability of the carbon radical intermediate that forms during the reaction. Carbons that have more carbon neighbors will more easily lose a hydrogen and form a carbon radical intermediate. The neighboring carbons, being larger than the neighboring hydrogen atoms, can help stabilize the formation of the carbon radical.
Thus, in the halogenation reaction tertiary carbons will be the most reactive positions, followed by secondary carbons and finally primary carbons.
Quaternary carbons are unreactive as they do not have any hydrogens available that can be substituted by a halogen. The reactions of the cycloalkanes are generally just the same as the alkanes, with hydrogen atoms on the cyclic ring structure being replaced by the halogen atom. For example i n the presence of UV light, cyclopropane will undergo substitution reactions with chlorine or bromine just like a non-cyclic alkane.
However, the small ring structures — particularly cyclopropane — also have the ability to react in the dark. In the absence of UV light, cyclopropane can undergo addition reactions in which the ring is broken. For example, with bromine, cyclopropane gives following linear compound. This can still happen in the presence of UV light — but you will get a mixture of the substitution reactions as well.
The ring is broken because cyclopropane suffers badly from ring strain. Why are alkanes sometimes called paraffins? Which halogen reacts most readily with alkanes? Which reacts least readily? Alkanes do not react with many common chemicals.
A wide variety of interesting and often useful compounds have one or more halogen atoms per molecule. For example, methane CH 4 can react with chlorine Cl 2 , replacing one, two, three, or all four hydrogen atoms with Cl atoms. Once widely used in consumer products, many chlorinated hydrocarbons are suspected carcinogens cancer-causing substances and also are known to cause severe liver damage.
An example is carbon tetrachloride CCl 4 , once used as a dry-cleaning solvent and in fire extinguishers but no longer recommended for either use. Even in small amounts, its vapor can cause serious illness if exposure is prolonged. Moreover, it reacts with water at high temperatures to form deadly phosgene COCl 2 gas, which makes the use of CCl 4 in fire extinguishers particularly dangerous.
Ethyl chloride, in contrast, is used as an external local anesthetic. When sprayed on the skin, it evaporates quickly, cooling the area enough to make it insensitive to pain. It can also be used as an emergency general anesthetic. Bromine-containing compounds are widely used in fire extinguishers and as fire retardants on clothing and other materials. Because they too are toxic and have adverse effects on the environment, scientists are engaged in designing safer substitutes for them, as for many other halogenated compounds.
Alkanes substituted with both fluorine F and chlorine Cl atoms have been used as the dispersing gases in aerosol cans, as foaming agents for plastics, and as refrigerants. Two of the best known of these chlorofluorocarbons CFCs are listed in Table 7. Chlorofluorocarbons contribute to the greenhouse effect in the lower atmosphere. They also diffuse into the stratosphere, where they are broken down by ultraviolet UV radiation to release Cl atoms.
These in turn break down the ozone O 3 molecules that protect Earth from harmful UV radiation. Worldwide action has reduced the use of CFCs and related compounds.
The CFCs and other Cl- or bromine Br -containing ozone-destroying compounds are being replaced with more benign substances. HCFC molecules break down more readily in the troposphere, and fewer ozone-destroying molecules reach the stratosphere. Ozone Depletion in the Upper Atmosphere. They occur mainly over Antarctica from late August through early October and fill in about mid-November.
Ozone depletion has also been noted over the Arctic regions. The largest ozone hole ever observed occurred on 24 September What is cracking? Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst , or lower temperatures and pressures in the presence of a catalyst.
A catalyst is a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil petroleum. These fractions are obtained from the distillation process as liquids, but are re-vaporized into the gaseous phase before cracking.
The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C 15 H 32 might be:.
Or, showing more clearly what happens to the various atoms and bonds:. This is only one way in which this particular molecule might break up. Notice that in addition to producing smaller alkanes, the cracking reaction can also produce alkenes with double bonds.
In this case, the alkenes — ethene C 2 H 4 and propene C 3 H 6 — are important materials for making plastics or producing other organic chemicals. The octane is one of the molecules found in petrol gasoline. An understanding of the physical properties of the alkanes is important in that petroleum and natural gas and the many products derived from them—gasoline, bottled gas, solvents, plastics, and more—are composed primarily of alkanes.
This understanding is also vital because it is the basis for describing the properties of other organic and biological compound families. For example, large portions of the structures of lipids consist of nonpolar alkyl groups. Lipids include the dietary fats and fatlike compounds called phospholipids and sphingolipids that serve as structural components of living tissues. These compounds have both polar and nonpolar groups, enabling them to bridge the gap between water-soluble and water-insoluble phases.
This characteristic is essential for the selective permeability of cell membranes. Tripalmitin a , a typical fat molecule, has long hydrocarbon chains typical of most lipids.
Compare these chains to hexadecane b , an alkane with 16 carbon atoms. To ensure that you understand the material in this chapter, you should review the meanings of the following bold terms in the summary and ask yourself how they relate to the topics in the chapter.
Organic chemistry is the chemistry of carbon compounds, and inorganic chemistry is the chemistry of all the other elements. Carbon atoms can form stable covalent bonds with other carbon atoms and with atoms of other elements, and this property allows the formation the tens of millions of organic compounds.
Hydrocarbons contain only hydrogen and carbon atoms. Hydrocarbons in which each carbon atom is bonded to four other atoms are called alkanes or saturated hydrocarbons. Any given alkane differs from the next one in a series by a CH 2 unit.
Any family of compounds in which adjacent members differ from each other by a definite factor is called a homologous series. Carbon atoms in alkanes can form straight chains or branched chains. Two or more compounds having the same molecular formula but different structural formulas are isomers of each other. There are no isomeric forms for the three smallest alkanes; beginning with C 4 H 10 , all other alkanes have isomeric forms.
Cycloalkanes are hydrocarbons whose molecules are closed rings rather than straight or branched chains. A cyclic hydrocarbon is a hydrocarbon with a ring of carbon atoms. Recall that a structural formula shows all the carbon and hydrogen atoms and how they are attached to one another. A condensed structural formula shows the hydrogen atoms right next to the carbon atoms to which they are attached.
A line-angle formula is a formula in which carbon atoms are implied at the corners and ends of lines. Each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds. The physical properties of alkanes reflect the fact that alkane molecules are nonpolar.
Alkanes are insoluble in water and less dense than water. Alkanes are generally unreactive toward laboratory acids, bases, oxidizing agents, and reducing agents. They do burn undergo combustion reactions. Alkanes react with halogens by substituting one or more halogen atoms for hydrogen atoms to form halogenated hydrocarbons. This reaction is called halogenation. An alkyl halide haloalkane is a compound resulting from the replacement of a hydrogen atom of an alkane with a halogen atom.
Larger alkanes can be broken into smaller alkanes and alkenes using high heat or catalysts. These reactions are called cracking reactions. It ignites readily and burns readily.
The substance is insoluble in water and floats on the surface. Is the substance likely to be organic or inorganic? What is the danger in swallowing a liquid alkane? Following is the line formula for an alkane. Draw the condensed structure. Write equations for the complete combustion of each compound. The density of a gasoline sample is 0. On the basis of the complete combustion of octane, calculate the amount in grams of carbon dioxide CO 2 and water H 2 O formed per gallon 3.
Draw the structures for four of the nine isomeric hexanes C 7 H Consider the line-angle formulas shown here and answer the questions. Text for this chapter has been adapted from the creative commons resources listed below, unless otherwise noted in the text. Chapter 7: Alkanes and Halogenated Hydrocarbons This text is published under creative commons licensing, for referencing and adaptation, please click here.
Opening Essay 7. Hacker Swallowed, liquid alkanes do little harm while in the stomach. Note The word organic has different meanings. Organic chemistry is the study of carbon compounds, nearly all of which also contain hydrogen atoms.
More Practice Classify each compound as organic or inorganic. Answers to odd questions: organic inorganic organic. NaOH KCl. Back to the Top 7. Examples of Alkanes Straight Chain Alkanes The straight chain alkanes, methane CH 4 , ethane C 2 H 6 , and propane C 3 H 8 represent the beginning of a series of compounds in which any two members in a sequence differ by one carbon atom and two hydrogen atoms—namely, a CH 2 unit Fig.
Concept Review Exercises In the homologous series of alkanes, what is the molecular formula for the member just above C 8 H 18? Answers C 9 H C 12 H 26 Back to the Top. 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 has been described. 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.
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