In a homologous series of organic compounds, the Auto ignition temperature (AIT) decreases with increasing molecular weight. Also a branched compound will have a higher AIT than the corresponding straight-chain compound. There is a plot for straight chain paraffins in one of the NFPA publications. The subject is discussed in Bodurtha's book on Industrial Explosion Prevention and Protection.

HAZOP at www.curryhydrocarbons.ca

The only probable exception to owg's statement is acetylene.

From the net:


AIT = Minimum Autoignition Temperature in ^oC under test conditions.

                                                   AIT
        n-butane                              370
        n-pentane                            260
        n-hexane                             225
        n-heptane                            225
        n-octane                              220
        n-decane                              210
        n-dodecane                           204
        n-tetradecane                       200

        2-methylpropane                    460
        2-methylbutane                      420
        2-methylpentane                    306
        2,2,3-trimethylbutane              420
        2,2,4-trimethylpentane            415

        cyclopentane                          380
        cyclohexane                            245
        methylcyclohexane                 250

        benzene                                560
        toluene                                 480
        ethyl benzene                        430
        meta-xylene                          463
        para-xylene                           530
        ortho-xylene                          530

Ethylene and acetylene auto ignition temperature needs clarification.  These two chemicals decompose into C +CH4 + H2, raising the pressure by a factor of three and giving off large amounts of energy.  Their auto decomposition temperature is a function of pressure.  Quite a tricky pair.