Identify hydrocarbons and alkene test results
Organic chemistry • Carbon compounds as fuels
Flashcards
Test your knowledge with interactive flashcards
Key concepts
What you'll likely be quizzed about
Definition of hydrocarbons
Hydrocarbons are molecules composed of only carbon and hydrogen atoms. Hydrocarbons appear in different classes such as alkanes and alkenes, defined by the types of carbon–carbon bonds present. Alkanes contain only single carbon–carbon bonds. Alkenes contain at least one carbon–carbon double bond, which causes different chemical behaviour in tests and reactions.
Alkanes: general formula and recognition
Alkanes follow the general formula CnH2n+2 for non-cyclic structures, where n denotes the number of carbon atoms. If a molecular formula fits CnH2n+2, the substance is an alkane unless the molecule is cyclic or contains heteroatoms that change the hydrogen count. Recognition of an alkane relies on the hydrogen count matching the general formula and on structural evidence of only single C–C bonds. Cyclic hydrocarbons and molecules with oxygen, nitrogen or halogens require adjusted formulas and separate identification.
Formula types: molecular, empirical, structural, displayed, condensed
A molecular formula states the actual number of each atom (e.g., C3H8). An empirical formula gives the simplest whole-number ratio (e.g., C3H8 has empirical formula C3H8 if no simpler ratio exists). A structural formula shows bonding between atoms (e.g., CH3CH2CH3), a displayed formula shows every bond and atom, and a condensed formula shortens groups (e.g., CH3CH2CH3). Recognition tasks require conversion between these forms: count carbons (n) and hydrogens, then compare with CnH2n+2. If the condensed or structural formula clearly shows only single bonds and the hydrogen count matches, classification as an alkane is valid.
Alkenes and the difference in formula
Alkenes contain one or more carbon–carbon double bonds and follow the general formula CnH2n for a single double bond in a straight-chain, non-cyclic alkene. A sample with formula matching CnH2n suggests an alkene rather than an alkane. The presence of a double bond changes reactivity: alkenes undergo addition reactions that saturated alkanes do not, producing clear outcomes in diagnostic tests such as the bromine water test.
Bromine water test: procedure and observation
Bromine water is an orange solution of bromine in water used to test for carbon–carbon double bonds. Addition of bromine water to a sample causes the orange colour to disappear if an alkene is present because the double bond reacts with bromine in an addition reaction. If an alkane is present under normal conditions, the bromine water retains its orange colour because alkanes do not undergo addition with bromine in the dark and at room temperature. The test therefore differentiates saturated from unsaturated hydrocarbons.
Limitations and false results
Some substances other than alkenes can decolourise bromine water (for example, phenols or compounds that oxidise bromine), producing false positives. Presence of catalysts, ultraviolet light or high temperatures can cause alkanes to react slowly with bromine by substitution, altering the expected observation. Clear identification requires considering sample composition and conditions. A single test should support classification but may require complementary analysis, such as infrared spectroscopy or chemical reasoning about molecular formula and structure.
Key notes
Important points to keep in mind