They are composed entirely of single bonds and are saturated with hydrogen. Saturated hydrocarbons are the basis of petroleum fuels and are found as either linear or branched species. The simplest alkanes have their C atoms bonded in a straight chain; these are called normal alkanes.
They are named according to the number of carbon atoms in the chain. The smallest alkane is methane:. Those with double bond are called alkenes and have the general formula C n H 2n assuming non-cyclic structures. Those containing triple bonds are called alkynes and have general formula C n H 2n The smallest alkene—ethene—has two C atoms and is also known by its common name ethylene and the smallest alkyne is ethyne, also known as acetylene. Cycloalkanes are hydrocarbons containing one or more carbon rings to which hydrogen atoms are attached.
The prefix "cyclo" is added to the name to communicate the ring structure. The general formula for a saturated hydrocarbon containing one ring is C n H 2n. Aromatic hydrocarbons, also known as arenes, are hydrocarbons that have at least one aromatic ring.
Aromatic compounds contain the benzene unit. Benzene itself is composed of six C atoms in a ring, with alternating single and double C—C bonds:. For most compounds, information beyond the chemical formula will be needed to elucidate their structure.
However, the ratio of C:H in a chemical formula can provide insights into the chemical structure. Toluene is similar to benzene, except that one hydrogen atom is replaced by a —CH 3 group; it has the formula C 7 H 8 part b in Figure 3. The chemical behavior of aromatic compounds differs from the behavior of aliphatic compounds. Benzene and toluene are found in gasoline, and benzene is the starting material for preparing substances as diverse as aspirin and nylon.
Figure 3. As shown, compounds with the same molecular formula can have very different structures. Write the condensed structural formula for each hydrocarbon. Show Solution. A The prefix hept- tells us that this hydrocarbon has seven carbon atoms, and n- indicates that the carbon atoms form a straight chain.
The suffix -ane tells that it is an alkane, with no carbon—carbon double or triple bonds. A The prefix pent- tells us that this hydrocarbon has five carbon atoms, and the suffix -ene indicates that it is an alkene, with a carbon—carbon double bond.
B The 2- tells us that the double bond begins on the second carbon of the five-carbon atom chain. A The prefix but- tells us that the compound has a chain of four carbon atoms, and the suffix -yne indicates that it has a carbon—carbon triple bond.
B The 2- tells us that the triple bond begins on the second carbon of the four-carbon atom chain. A The prefix cyclo- tells us that this hydrocarbon has a ring structure, and oct- indicates that it contains eight carbon atoms, which we can draw as. The suffix -ene tells us that the compound contains a carbon—carbon double bond, but where in the ring do we place the double bond? We can draw the structure of cyclooctene as.
Show Answer. The general name for a group of atoms derived from an alkane is an alkyl group. The name of an alkyl group is derived from the name of the alkane by adding the suffix — yl. Thus the —CH 3 fragment is a methyl group, the —CH 2 CH 3 fragment is an ethyl group, and so forth, where the dash represents a single bond to some other atom or group. Lesson: 4c. Intro Learn Practice. Do better in math today Get Started Now.
Introduction to organic chemistry 2. Alkanes 3. Alkenes and unsaturated hydrocarbons 4. Alcohols 5. Haloalkanes 6. Benzene and aromatic compounds 7. Naming organic compounds and groups Back to Course Index. Don't just watch, practice makes perfect. Lessons Notes: In this lesson, we will learn: The definition of an alkene and their general formula. The major uses and properties of alkenes.
How to test for alkenes in a chemical reaction. Notes: We saw in Alkanes that alkanes are saturated hydrocarbons , compounds made of only carbon and hydrogen atoms where carbon makes only single bonds. However, many organic compounds are unsaturated. This means that not all the bonds made by carbon are single bonds, they also contain double or triple bonds , to either carbon or another atom.
Like alkanes, alkenes are another homologous series of hydrocarbons. Alkenes are unsaturated hydrocarbons with one or more carbon-carbon double bonds. Alkenes have the general formula: C n H 2n. This means that in a simple alkene only one double bond there are twice as many hydrogen atoms as there are carbon atoms.
Alkenes are more reactive than alkanes because their double bond s can be opened up by chemical reactions; it is a more reactive bond than a single bond.
This means alkenes are very useful for making polymers , which are very long chains of hydrocarbons made by a repeating unit. This is especially true of ethene, the smallest alkene, which is the monomer unit of the important plastic polymer poly ethene.
Alkenes can make two new bonds with other atoms by opening up this double bond. This is where the terms saturated and unsaturated come from. Like a sponge saturated by water, an alkane is saturated by bonds; it cant form any more bonds, but alkenes can so it is unsaturated. Alkenes produce more soot when burning than alkanes do, which have a cleaner flame.
Practically, this just means it requires more oxygen to burn cleanly because there are more C-C bonds with the double bond present to have to break up. You can use bromine water to test for alkenes : When an alkene solution is added to bromine water, the brown color of the bromine solution will go colorless.
We say that alkenes decolorize bromine water. The brown color caused by bromine water disappears because bromine Br2 is being reacted away. The double bond in the alkene molecule reacts with a bromine molecule and opens up in an addition reaction, using both reactant molecules up. A colorless dibromoalkane product forms in their place.
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