in this video we will explore two concepts in organic chemistry relating structure and formula namely homologous series and isomers playing with shapes can teach us a lot about these two concepts very simply homologous series are families of organic compounds in which the molecules have the same general formula and similar structures but their lengths give them different molecular formulas very simply isomers are organic compounds with the same molecular formula but different structures let's investigate each of these two concepts in more depth starting with homologous series using the molecules in the table as an example we can see that a homologous series is a group or sequence of compounds with the same functional groups in the case none since the molecules are all fully saturated hydrocarbons and thus similar chemical properties and the same general formula but different chain lengths and thus different molecular formulas each successive compound in homologous series differs by a C2 unit the examples in this table which are methane ethane propane and butane form part of a homologous series of alkanes the alen alkin and alcohols are other examples of homologous series although compounds in a homologous series have similar chemical reactivities they usually show Trends in physical properties such as melting point boiling point and solubility for example in the carboxilic acid homologous series and many other homologous series with increasing carbon chain length so there is increasing melting point increasing boiling point and decreasing solubility in water now let's turn our attention to isomers the atoms and bonds in an organic compound can be arranged in different ways the resulting compounds have the same molecular formula in this example all three compounds have the molecular formula c5h12 but they have different bonding patterns so these are different compounds with different properties these compounds are called isomers if isomers have different Arrangements of carbon carbon and carbon hydrogen bonds in other words different hydrocarbon backbones they are called chain isomers these three compounds are chain isomers since they can be produced by changing the size and positions of side chains if two isomers contain the same functional groups in different positions they are positional isomers for example butan 2 is a positional isomer of butan 1 o here's another example pint 1 in and paint two in are positional isomers of each other if two isomers contain different functional groups they are functional group isomers functional group isomers must have the same number of multiple bonds so only certain pairs of functional groups can be related in this way ethers may be isomers of alcohols ketones may be isomers of alahh and Esters may be isomers of carboxilic acids to fully understand related isomers we need to understand the arrangement of carboncarbon bonds in a compound a carbon atom can be described using the terms primary secondary and tertiary using alcohols as an example if a hydroxy group is attached to a primary carbon atom that is a carbon atom bonded to only one other carbon atom the compound is a primary alcohol butan 1 is a primary alcohol similarly a secondary alcohol contains a hydroxy group attached to a secondary carbon atom a secondary carb carbon atom is bonded to two other carbon atoms butan 2 is a secondary alcohol a hydroxy group on a tertiary carbon that is a carbon atom bonded to three other carbon atoms gives a tertiary alcohol two methy propane 2 is a tertiary alcohol these compounds are isomers since they have the same molecular formulas the same descriptors can be used for halogeno alkanes for example chloropropane is a primary chloro alcane while two chloropropane is a secondary chloro alcane these compounds are isomers in this case there is no corresponding tertiary isomer amines are described slightly differently in these compounds the the labels primary secondary and tertiary indicate the number of carbon atoms bonded to the nitrogen atom for example this compound is a primary Amin this is a secondary amine and this is a tertiary Amin now let's summarize the main points a homologous series is a series of compounds with the same functional group and thus similar chemical properties the same general formula but where each successive compound in the series differs by a ch2 unit compounds in a homologous series display Trends in physical properties according to carbon chain length isomers are compounds with the same molecular formula but a different arrangement of atoms chain isomers have different hydrocarbon backbones positional isomers have the same functional group but in a different position and functional group isomers have different functional groups finally in a compound a primary carbon is bonded to one other carbon atom a secondary carbon to two other carbon atoms and a tertiary carbon to three other carbon atoms s